Well I'm not going to tell you to hit the weights considering your age but you can do cardio and eat right to make a difference. Try to do cardio four times a week at 30 minutes. You are in school so your best bet would be after. Cut out all your fats and sugars. You got to start eating every 3 hours to get your metabolism going. Have very small meals. Here are the foods to eat: skim milk, fruits/vegtables, egg whites, fish, red meat, chicken, etc.... Seeing as your only 13 you are proabably lost with all this info but that's the best I can put it. You might want to actually talk to your doctor.