You'll have to cut your daily calories and eat healthy foods - talk to your parents about making sure there are plenty of veggies, lean protein, etc available.
Try walking (or other activity you enjoy) and yoga for exercise. Walking burns fat, as long as you keep a good brisk pace for at least 30-60 min. Yoga helps build muscle (which burns fat) and is great for toning. You can do a google search for different yoga videos, that you can watch for free, in order to learn how to do it. Don't worry if you can't do as well as the person in the video -- they've been doing it for a long time; do the best you can and you'll realize that as you do it, you get better.
Remember...you can NOT target reduce. Meaning if you do crunches your belly will not get smaller. Proper nutrition and a balance of intake calories with exercise should give you results. Just a tip...you need 10 calories for every pound to maintain your current weight. But dont go crazy with this, when the bodysees a reduction of by more than 10-15% of calories needed it freaks out and stores fat. Slow and steady...
Best of luck