If you don't eat breakfast in the morning that could make you fat. Eat a really big breakfast in the morning, a smaller lunch and a very very small dinner. Try to avoid eating pizza for diner because it has a lot of cheese, and cheese has fat in it. Avoid eating fat at the end of the day.
Honestly I had the same problem. Your still young and your body is growing. You can eat healthier and be more active, but don't force it because your still very young. Also if you drink a lot of sodas try water. (Cutting out sodas helped me A LOT!)