Check your weight on weight-height table. You can get it by searching on the Internet. Here are some suggestions.
Avoid much soft-drink and have a healthy breakfast with fruits. And avoid having a nap in midday.
Get lots of sleep. In order to maintain a healthy body, you'll need 8-10 hours of sleep every day. Go to sleep earlier on school nights.
Get a little exercise every day. This will not only make you feel better, and make you look better, but help you to get through the day.
Five time prayers will keep you fit both mentally and physically.