This page lists all of the Xbox 360 Achievements, plus their gamerscore and how to get them.
Learnt 5 skills-5 gp: Simple, just learn 5 skills.
Learn 10 skills-10 gp: Again, learn 10 skills.
Learn 20 skills-20 gp: Quite hard. You will be about level 15 when you get this achievement.
Learn all skills-50 gp: Hard. Learn every skill in the game. Visit the trainers. Start this queast early, as the price for trainers gets very high when you aproach the higher levels.
Reach skill level 10-20 gp: Learn a skill to maximum efficency.
Reach level 5-5 gp: Get your character to level up to level 5.
Reach level 10-10 gp: Again, reach level 10 with your character.
Reach level 20-10 gp: Reach level 20.
Reach level 35-20 gp: Reach level 35.
Reach level 50-50 gp: Eventually reach character level 50.
Discovered 10 locations-10 gp: Easy, find 10 different locations eg caves, cities, dungeons e.t.c.
Discovered 20 locations-10 gp: Again, find a total of 20 locations in game.
Discovered 50 locations-20 gp: Find 50 locations.
Visit all Black Towers-5 gp: Find every black tower which is in the continent.
Visit all the Graveyards-10 gp: Find all of the graveyards around the game.
Visit 10 undergrounds-10 gp: Find and enter 10 of the undergrounds in the game eg dungeons, underground caves, cellars e.t.c.
Visit 20 undergrounds-20 gp: As above, find and enter 20 of the underground locations in game.
Visit all undergrounds-20 gp: Find and enter all the undergrounds.
Visit all locations-50 gp: Find every location in game. Very hard.
Ride a horse-5 gp: Easy. Find a horse and get on it. You don't even need to ride it.
Make successful teleport-5 gp: Once you have finished the Old Friend 'Quest', you can use teleporters. Use one to get the achievement.
Create a potion-5 gp: Very easy. Get 2 ingrediants, select them then activate the pot. Potion made.
Create a perminant potion-20 gp: Easy-Quite Hard. You will need 2 permanent ingredients. If you have those, it will be easy.
Create a bomb-20 gp: For this achievement, you will need 2 temporary effects mixed together.
Create an weapon enhancer-15 gp: Find an elemental gem, then use it with an ingrediant.
Create a Class 2 item-5 gp: Basically, get 2 identical items. If when you highlight one the other is green, you have done it right. Tap Y, select combine and then combine it with the other item.
Create a Class 10 item-15 gp: The same as above, but you will need to repeat this 10 times. To find identical items, kill bandits and take their boots. They are normally the same.
Open a master lock-10 gp: For this, you will need a lot of patience, lock picks and a master lock. It helps to have a high lockpicking skill (about 7/8). There are lots of locked doors when Ashos gets attacked by orcs.
Killed a Stone Gollum-10 gp: Easiest in Hadeborg, but they are tough. When they are about to strike a blow, tap B to jump back.
Use a boosted spell-5 gp: Find or buy a booster card. Equip it in you spell amulet, then cast the spell. The spell doesn't need to have any effect.
Cast a Chamber 5 Spell-10 gp: Chamber 5 Spells are the most powerful spells in the game. Get a level 15 in the selected school of magic, buy a chamber 5 spell, equip it, and then cast it.
Found the Relic Frame-20 gp: Complete the main quest up to this point.
Found the Earth Element-20 gp: Complete this part of the main quest. It is recommended that it is done first.
Found the Water Element-20 gp: Complete the main quest up to this point. It is recommended that this is done second.
Found the Air Element-20 gp: Do this part of the main quest. This should be sone third.
Found the Fire Element-20 gp: Finish this part of the main quest. Do this last.
Deliver the Relic-20 gp: Deliver the frame and elements to Qudinar.
Destroy the Great Pentagram-25 gp: Destroy the Great Pentagram as part of the main quest.
Defeated the Great Enemy-370 gp: Finish the main quest.
A more detailed discussion on how to get achievements can be found here.