why not just eat and exercise for health - and see what happens to your body.  Ask your mom and dad to walk with you - jog with you or provide you with some weekly exercise opportunities after school (maybe you can train together at a gym, or take a class) and on weekends start biking or doing some other fun activities.

Reduce your "processed food snacking" - start having fruit, fat free yogurt, veggies and a bit of healthy non fat dip for snacks.  Brown bag food from home ot school and create healthy choices.  Start your day with a portion controlled healthy breakfast - cereal/skim milk/fruit or a whole grain waffle -teaspoon of peanut butter - berries or a healthy smoothie or yogurt/nuts/fruit.

Let your family know how important this is to you.  My book Fat Families Thin Families might be a great read for you and the family too.