If there's any way you could go for a run after school, do it. It might be something like laps around your house, but do SOMETHING.
Running can help get rid of extra leg fat (if you have any) and define your muscles. I would see if cross country is my sort of thing first, then see about after school jogs.
In a nutshell, you should just exercise a lot. If you want a diet like these other answers, that's okay too, but I got away with running every day for two months. It really shows!