Leveling system

I’m looking for a tutorial on creating a leveling system with a level progress bar like you would find in World of warcraft. I want it so the bar fills as you gain exp and when you level the bar emptys and fills all over again for the next level reqireing the new exp. Is there any tutorial or anything any one know of for this. I created one but I cant get the exp bar to to fill fill up then change for the next level. I am only look like 10 level to start with.

Creating a progress bar that fills is extremely easy with NGui. They have a free version, I suggest taking a look there.