It sounds like you're still going though puberty, which means you'll probably grow into the fat you have over the next few years. I also wouldn't worry about a being a "bit fat". As long as you're not morbidly flubby, ugly, and smelly, being a little big isn't really that bad. There's four things I do to keep in shape and keep the weight off. If you have any interest, martial arts are a great way to get extra exercise and lose body fat. In addition to martial arts, get a gym subscription and hit the weights. Muscles eat calories. Thirdly, join an organization that involves exercise. You're old enough to join military auxiliaries such as Civil Air Patrol and Coast Guard Aux.

My main point is that its good to find a form of exercise that's enjoyable. Diet is also key to losing weight. Best of luck.

also just a tip - a lot of people do not know that with weight training, a large amount of reps of light weights will tone while a small amount of reps with heavy weights will build muscle.