Finish the main quest. Now, I recommend you start out at Casual. No, Normal is NOT Hardcore, and Veteran is NOT Insane. So, go easy, find out where everything is at. SAVE YOUR MONEY for the next play through. Make sure: You use two allies always! Do ALL side quests with them. Don't worry, those unlock later on, some times different times in the game.

Feros is the planet that has the colonists and is not covered in snow. Now, all you have to do is: Talk with the leader[Dai Fan, I think was his name], kill the Geth, now this is important: Talk with him again, then talk with the people in charge of: Food, water, and power, and do their side quests! After that, move up the bridge, go to ExoGeni, find Lizbeth, return to the colony. Now, either upgrade your grenades with the Thorian gas or kill them all. Now, if you kill them and didn't do the side quests, you will NEVER complete them.

This is your first mission. This is important: IF you want insane or hardcore, you CAN CHANGE the difficulties in game. DO IT BEFORE LANDING ON EDEN AND/OR passing it! This was confirmed in the Bioware Forums. Now, just kill the Geth then de-activate the bombs by standing near them and holding A.
After that, approach the Beacon.

CONTAINS SPOILERS!!!

Virmire is the planet you receive from the Council. First, you land, clear the AA guns and approach the Salarian Camp. You can get the Charm/Intimidate on Wrex, who will want to get the cure for the Genophage. Don't worry if you don't have it maxed out, there's another chance later in the game. Now, choose between Kaidan or Ashley to go with the Salarians. SPOILER!!!: Later in the battle, you will have to choose who to save. IF one dies and you're doing the romantic sub plot, it will end there.

Noveria is the planet covered in Snow, and has Matriach Benezia. In order to get the garage pass you need to help Lorik. Go to the garage, kill the Geth, and drive up. Once in the facility, you'll need to activate the VI's core. Either doing the Tower of Hanoi puzzle[search in the site or google for how to do it] or sacrifice 100 omni gel. Then, activate the power and landlines. Once in the tram, you must kill the Rachni, but first go down the elevator to where the survivors are at, get the pass, and to the hot labs. Activate the Purge and get out. Return to the survivors, [SPOILER] Kill them. Go to the end of the place, and go to the tunnels. Find the Matriarch. To kill her, you MUST kill the Geth and Asari Commandos. DO NOT FIGHT HER. Start off, kill the Asari Command rushing you, then RUN TO THE RIGHT and kill the Geth and Commandos after talking with her, she'll go berserk again. Kill ALL enemies, then kill her.

After finishing your Casual, or any play through, select new career and existing ID. Make sure to change the settings in the main menu, difficulty to Hardcore. WHEN YOU START THE GAME IN THE NORMANDY, CHANGE IT AGAIN TO HARD CORE! IF YOU LAND ON EDEN PRIME AND DIDN'T CHANGE IT, YOU WON'T GET THE ACHIEVEMENT!!!!!

ALL organic enemies have immune. They are very accurate and WILL cut you to shreds. Take cover often, but watch out for rockets. Use the Spectre Master Gear and a lot of Biotic powers. Lastly, all enemies are the same level as you.

Refer to the Long Service Medal achievement.

Refer to Distinguished Combat Medal and Long Service Medal. Note: ALL organic enemies have immune. They are very accurate and WILL cut you to shreds. Take cover often, but watch out for rockets. Use the Spectre Master Gear and a lot of Biotic powers. Oh, and for fun, enemies and bosses are higher levels than you.

Alright, start with the pistol, since it's the hardest weapon to get kills and the weakest. Register 150 kills, do side quests, for more enemies. Don't worry, kills carry one through careers. Also, YOU must make the kills, your allies can't help you here. Make sure you level the weapon up for more damage. Also, be realistic, if you haven't leveled your gun up and you're facing a Krogan, switch weapons.

Leave the shotgun for last, or third, if you want. Upgrade the weapon for more damage. Now you ask, why for last? In the last missions, enemies will be close together, and with a high level shotgun, you can basically snipe them. IF you don't get the 150 kills, don't worry, they carry on to the next career. YOU MUST do the kills, ally kills will NOT count.

This should be the weapon you use the most in the game. Once it is fully powered, you can destroy Geth Colossus and Gun turrets with the over kill power up. Again, they carry on to the next play through. YOU must make the kills, ally kills will NOT count.

This is the most annoying weapon in the game. If you're not leveled up, it will be hard to get kills with this. Make sure this is your first or second weapon you use, since it's hard. YOU must make the kills, ally kills will NOT count.

Create an Adept character or any character with biotic powers. Use the power on a wall. Also, make sure you have adrenaline rush. Use all the powers, adrenaline rush, repeat all powers, and wait for them to recharge. YOU must do them, allies don't count.

For this achievement and the stasis one, you HAVE TO BE in combat. Use the abilities, adrenaline rush, use them again, kill enemies, let them recharge. IF you do ALL side quests you should get it before finishing the game.

For these achievements, you'll need to be an engineer class character. Simply use these by pressing the RB and highlighting the power. MUST be done on an enemy. Make sure you have adrenaline rush. Use ALL abilities then use adrenaline rush in order to do them again without waiting. Also, YOU must do then, allies don't count.

For this achievement, all you will need to do is press the Y button. Make sure you're damaged, or else it won't allow you to use the medi-gel. This carries over play any through. It is likely you'll get it after finishing Hard Core.

Note: You only get ONE chance to get MOST of these.

The Council consists of: Asari, Turians, and Salarians.
The extinct Races are: Protheans, Rachni
The Non- Council races are:Batarians, Elcor, Geth, Hanar, Keepers, Krogan, Quarians, Volus

Turians:
At the very beginning of the game, talk to people about the Spectres and Saren.
Asari:
Either talk with Captain Anderson, or with Avina[The VI about the council races].

Salarians:
Talk to Captain Anderson after the first meeting with the council.

Protheans:
CAN ONLY GET WITH TALKING WITH CAPTAIN ANDERSON!

Rachni:
Talking with the VI near the Krogan Statue about the Krogans, the rebellion, and the war.

Batarians:
After acquiring the Normandy, talk with Captain Anderson. Ask him about him becoming a Specter.

Elcor
Talk to the Elcor diplomat Calyn in the embassy. The embassy is to the right of the Human Embassy.

Geth:
Ask Ashley AFTER finding her on Eden Prime.
.
Hanar
Either with the Preaching Hanar, or with the Shop Keeper that is Hanar. Ask him about his speech.

Keepers:
Ask Avina near the Citadel Tower light.

Krogan:
Automatic by watching the confrontation between Wrex and the Krogan bouncer in Chora's Den.

Quarians:
Talk to Garrus in medical center.

Volus:
Talk with Avina outside the embassies.

By majority of the game, they mean an undefined amount of the game. I recommend that you choose two characters and go with them till the end. Try to not start a side quest without the characters you want. There around 70 side quests including the main quest ones. So, make sure you do around 60 of them before Ilos. IF you haven't gotten the achievement after that, then you should try exploring the galaxy and doing more quests. Remember to talk with allies in order to generate more side quests. The following is a list of missions in the game:

NOTE: I got completionist and both ally achievements by doing ALL side quests[not including Styx Theta Cluster ones, which are two] and completing the three main planets. Should be noted that I got the last ally Garus right when......MASSIVE, UBER SPOILER!!!!!!!!

SPOILER IS RIGHT UNDER THIS:

Saren committed suicide.


SPOILER OVER!

MAIN QUEST:

Prologue: Find the Beacon

Citadel: Expose Saren
Citadel: Garrus
Citadel: Wrex

Feros: Save the colony

Noveria: Matriarch Benezia
Noveria:Lorik Qui'n
Noveria: Peak 15
Noveria: Reactor Repair
Noveria: Reconnect Landlines
Noveria: Contamination
Noveria: The Hot Labs
Noveria: Rift Station

Find Liara T'Soni

Ilos: Find the Conduit

Race Against Time


SIDE QUESTS:

Citadel: Scan the Keepers
Citadel: Presidium Prophet
Citadel: Reporter's Request
Citadel: The Fan
Citadel: Rita's Sister
Citadel: Signal Tracking
Citadel: Jahleed and Chorban
Citadel: Homecoming
Citadel: The Fourth Estate
Citadel: Family Matter
Citadel: Asari Consort
Citadel: Old Friends
Citadel: Xeltan Convinced
Citadel: Planting a Bug
Citadel: Schells and his Data
Citadel: Negotiator's Request

UNC: Missing Survey Team
UNC: Rogue VI
UNC: Hostage
UNC: Major Kyle
UNC: Distress Call
UNC: Missing Marines
UNC: Privateers
UNC: Hostile Takeover
UNC: Matriach Writings [10 needed, I collected 15]
UNC: Missing Scientist
UNC: Turian Emblems [13 needed, I collected 17]
UNC: Minerals [ Rare Earths: 20, I found 23. Gases: 6, I found 10. Heavy Metals: 20, I found 25. Light Metals: 20, I found 22]
UNC: Signs of Battle [10 Medallions, I found 10] [ID Tags: 3, I found 4]
UNC: Negotiations
UNC: Cerberus[SHOULD BE NOTED: There is ONE other side quest to do with them on the Styx Theta Cluster, does NOT appear on Journal, search for abandoned ship].
UNC: Prothean Discs[7 needed, I found 10]
UNC: Lost Freighter
UNC: Espionage Probe
UNC: Hade's Dogs
UNC: Lost Module
UNC: Geth Incursions
UNC: ExoGeni Facility
UNC: Colonists
UNC: Derelict Freighter
UNC: Listening Post Alpha
UNC: Listening Post Theta
UNC: Depot Sigma-23

Virmire: Wrex
Virmire: Kirrahe's Team

NOTE: DO ALL FEROS BEFORE LEAVING TO THE SKY BRIDGE!!!

Feros: Recover Data
Feros: Power Cells
Feros: Varren Meat
Feros: Feth in the Tunels
Feros: Water Restoration


Noveria: Espionage

Asari Diplomacy


ALLIES:

Garrus: Dr Saleon
Wrex: Family Armor
Tali's Pilgrimage

Simply complete the game with YOU having more shield damage than health damage. Take cover often, not much else I can say.

Explore the galaxy! I believe I got this when I landed on the moon/planet that has Major Kyle. Don't worry, you'll eventually land on a planet that you need to do a side quest on.

Yes, credits carry on to the next game. Do all side quests. You get more money being Renegade, push for more credits, sell stuff. You can either save the money in your first play through in order to buy the Spectre Master gear quickly or save it for the second play through. Up to you.

Add the Shredder bullet upgrade to your weapon and allied weapons. Kill anything that is NOT Geth. You'll get it eventually. Yes, they carry one. Do side quests. The Cerberus missions have a lot of mercenaries.

Add the Tungsten upgrade to your weapon. By Synthetic, they mean Geth. IF you don't get this by Virmire, don't worry. The last mission has A LOT of enemies. Do side quests, there are plenty of Geth Ambushes in the beacons.

See completionist achievement. Do A LOT of side quests. Don't worry if you get one ally first and later the other. I got Ashley when I did ALL side quests [except two] and when I... SPOILER!!!!!!!!!!!!!!!!
SPOILER!!!!
killed Benezia.

I got Garrus when I DID ALL SIDE QUESTS!!! AND SPOILER!!!!!!!!!!!
SPOILER!!!
When Saren killed himself.

Do a lot of side quests. If you do them all, by the first play through you'll probably get to level 40. There is a glitch that only gives you 300xp every time, but to do it: Find Liara when SHE IS TRAPPED IN THE BLUE FIELD! Kill the enemies, save, then load the save. Congrats, you got 300 points! I would only recommend that in the early levels. Also, make sure that when on the Mako to kill enemies on foot, not with the Mako, you'll get more experience that way.

YOU MUST FINISH THE GAME ONCE in order to unlock levels 51-60. In total, you will need: 862100 experience points to reach level 60. Do ALL side quests, and it is most likely that you will get this at the end of your second way through or third. Also, make sure that when on the Mako to kill enemies on foot, not with the Mako, you'll get more experience that way.

On Noveria, when you are given the oppurtunity to accept the assignment from Lorik Qui'in to get the evidence to catch the Salarian (Anoleis, the administrator guy who has his own office) in his rend-based scandal, agree to it. He will open up the shut-down elevator, go down there and get the evidence. Upon exiting to go back to him you will run into Gianna (receptionist lady) talk to her then go meet her in the hotel/bar, she will ask you to get him to agree into turning Anoleis in, agree and go talk to Lorik Qui'in. Talk to him, choose the following options :

(Top Right) Testify against Anoleis
*Paragon or Renegade point choice, this is where the
6 in charm/intimidation comes into play*
(Top Left) {Paragon} You'd be a hero
(Bottom Left) {Renegade} Damn right I will

Talk to him again :

(Left) Matriarch Benezia
(Bottom Right) Another Question
(Top Left) Testify against Anoleis
*Paragon or Renegade point choice

Repeat the Matriarch Benezia steps to keep getting +25 point's, keep doing it untill achievement unlocks.

^ Thanks to Gprime for better explaining.

You can do this with: Kaidan, Ashley, and Liara. Talk with them after every main plot event. Choose the top dialog. Don't worry if you try to get Ashley and Liara[you dogs XD], they will confront you and you have to choose. Once you finish the three planets, and reach the Citadel, you will be close to kissing, and when heading to Ilos, you seal the deal.

This is a main quest achievement. Prove that Saren is evil. Talk with Harkin, find Garus, then bring down Fist. After that, you need to save Tali, and with her evidence, you become a Spectre.

CONTAINS SPOILER!!

You get two chances to do this. The first is on Virmire, where you either intimidate or charm Wrex into calming down and NOT taking action. The other chance is in the end where you intimidate or charm [SPOILER]
SPOILER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Saren into suicide.

You NEED to max out either one, or else you will fail.

She's in the Artemis Tau Cluster. Knossos system, and on the only planet you can land on. Drive the Mako till you reach the tunnels. Go down the elevator, and you'll find her, trapped in a force field. Kill the enemies, and activate the laser. Enter the newly made hole, use the elevator, and kill the Krogan.