How to shed some pounds before school starts?

I am not fat, I am not super-skinny either, I am healthy and muscular. I am toned, and I have a hard body, but I really need to stop eating junk-food. I am really worried because I got hurt just before school got out and will have to skip the fall/winter sports seasons, I am worried that I will not be able to burn off most of what I eat, I am looking to change my dietary lifestyle, not just loose a few pounds (but that doesn't hurt). I am thirteen.