How To Lose Weight Fast For A Teen?!

Count your calories and don't eat greasy food, fried food, fast food, salty food, sugary food, or food with a lot of sauce.
Don't eat any sauces or dressings, like on salad. If you need dressing use low-calorie dressing. Eat fish {not fried}, grilled chicken, fruits, veggies, and drink PLENTY of water. It will flush all the bad stuff out of your body.
Grapefruit really helps. If you eat a piece in the morning every day it will kick your metabolism into gear. Never skip breakfast, but eat right. Have a handful of nuts and/or berries, a cup of Cheerios or Special K, healthy stuff like that.
Exercise EVERY DAY, or at least 5 days of the week. Run for an hour, ride your bike for an hour, dance for an hour, whatever. You can do 200 jumping jacks to warm you up and burn calories (it's not that hard).
Also, here's another tip. And this won't turn you into an anorexic or anything--fast{don't eat anything} for 2-3 days. Drink water and if you must eat, eat veggies. It prompts your body to clear the gunk out of your insides, getting everything nice and polished around there. But don't fast for too long because then your body goes into starvation mode and starts eating itself. Yikes!
Goodluck xx