Eat less junk, and eat more veggies.Exercise a bit every day, or if not, every few days. Push yourself jogging, running, or playing a sport you like.
You seriously don't need to count calories at your age, and besides, you're still growing. So just try to eat healthier overall, not anything drastic. (For example, if you have a chocolate bar everyday, or chips everyday, replace them with some fruit, or carrots+hummus, etc.)