Here is the full list of all 45 Final Fantasy VIII achievements. It takes around 100-120 hours to unlock all of the achievements on Windows.
Complete the Obel Lake side quest
Reach maximum Gil
Reach level 100
Unlock Guardian Force Doomtrain
Reach maximum HP
Unlock Guardian Force Bahamut
Complete the PuPu side quest
Unlock Guardian Force Cactuar
Find Ragnarok
Collect all Weapons Monthly/Combat King/Pet Pals/Occult Fan magazines
Collect all Timber Maniacs magazines
Unlock Guardian Force Alexander
Unlock Guardian Force Cerberus
Acquired an item from Chocobo World
Unlock Guardian Force Pandemona
Upgrade your weapon
Capture a Chocobo
Reach SeeD rank A
Unlock Guardian Force Tonberry
Defeat every member of the CC Group
Unlock Guardian Force Leviathan
Win 100 games of Triple Triad
Lose a rare card in a Triple Triad game
Draw (Stock) magic from enemies 100 times
Unlock Guardian Force Carbuncle
Unlock Guardian Force Brothers
Kill 100 enemies
Unlock Guardian Force Diablos
Get your first salary from SeeD
Unlock Guardian Force Siren
Unlock Guardian Force Ifrit
Play Triple Triad
Unlock Guardian Force Shiva
Unlock Guardian Force Quezacotl
Unlock Guardian Force Eden
Collect all cards
Finish the game
Finish the game with Squall's initial level
Beat the Omega weapon
Draw magic 100 times from draw points
Reach level 100 with Boko
Learn all of Quistis' limit breaks
Learn all of Rinoa's limit breaks
Kill 1000 enemies
Kill 10000 enemies