Two suggestions -
(a) Invest in Nutrition for Dummies and follow the recommendations there for portion control, best foods from all the food groups. You don't say how old you are so i'm going to assume you can do a walking program before or after school or ask your parents for a gym membership if you are old enough. It is sticking with an exercise program for as long as a year that may help you to lose belly fat specifically. Weight training to redistribute your weight will also help to nudge overall weight loss. The Firm has a great exercise DVD series - kickboxing might also help - and especially target your belly or core muscles.
You need mom and dad to support you and I recently wrote a book - Fat Families Thin Families, which asks your family to come together as a "family team" and do this together. It sounds like you can all benefit.