Each class has its unique set of abilities.
Class abilities are skills used mostly in combat. You must have few rules in your mind when picking them. Most of all, before you choose an ability, learn its description and what does it do. Think what sort of character you want to develop. You should have a plan for your character from the beginning of the game. If you know what you want to achieve, it is much easier to choose abilities. Your character should create a coherent whole. Plan ahead. Pick effect that your character doesn't already have, avoid duplicates.