Servant has a good point.
the first thing you have to do is decide what kinds of experience points you'll have in the game, and what they represent experience in. D&D started with exp representing experience in combat as a certain class of character.
nowadays it seems folks forget its supposed to model something, and just use it in whatever manner, sometimes awarding "experience points" for all kinds of things, from follower kills, to location discovery, to quest completion. whats this supposed to represent? experience in playing the game?
experience points are not a game mechanic like jumping or powerups in a side scroller. they are a way of modeling something in a simulation. an often abused way.