Provoke puts you at 1 enmity "point" (let's call it that) above the person who has the most aggro on a certain mob - this should always be used by another skill, preferably one that generates a lot of enmity (ideally your ranged skill or the third skill of your enmity generating combos) to ensure you grab firm hold of that mob.

This skill is mostly used
1) For tank swaps in end game
2) If a dps or healer accidentally steal aggro from you and you need to get it back

EDIT: At lower levels there's not that many skills to keep track of, and you only gain new skills every couple of levels. You have time more than enough to learn what each skill does at a nice pace and you'll be good to go when you hit 60