How do you become apprentice in cabal

press "K" in game. It brings up skill window, there you can see all skills you have bought or received in any other way. You will also see you skill rank (novice, apprentice, etc.) and skill level (1/20 for example) in there.

Every time you use a skill, you receive skill experience. After using skills enough times, you will get skill level. after all skill levels of rank have been filled (20/20 for example) you move to next skill rank.