If you're 15 your body is going to change naturally, based on age and how you're growing. Don't worry about loosing weight at this point, just focus on eating right and keeping fit. When you've completely finished growing, that is when you can start to shed the pounds.

The problem is that when your growing and trying to diet you are going to mess with important fat cells, like your brain, and your boobs. If you diet when you're young you are going to live a life of flat chestedness, which you'll regret later.

Just focus on being healthy at this point.