As soon as you hit level 15 and are either dark- or light-side aligned talk to Kreia and she will give the choice of one of two sets of three prestige classes based on your alignment and extending and enhancing the abilities of one of the core classes.
For light side they are:
Jedi Master (extension of Consular)
Jedi Watchman (extension of Sentinel)
Jedi Weapon Master (extension of Guardian)
For dark side they are:
Sith Lord (extension of Consular)
Sith Assassin (extension of Sentinel)
Sith Marauder (extension of Guardian)
Pick one and the achievement will unlock