What to do if I'm a teenager and I'm approximately 11.4 ponds overweight. What is the best way to lose about 10 pounds fast?

Just be sensible. The real key to weight loss is to just watch the size of your portions at meals, eat healthy, and exercise regularly. There's no radical diet or exercise plan or pill that will magically give you meaningful and sustained weight loss. The ol' fashioned way is still be best way to lose weight for the long term. Shoot for no more than two pounds weekly.
Not fast. Talk to your parents before dieting. To lose weight insure that calories burned > calories consumed. Drink at least 8 - 8 oz glasses of water per day. Try for 8 hours of sleep/night. Aim for at least 5 servings of vegetables & 4 servings of fruit per day. Use whole grain when possible. Avoid sodas, alcohol & junk carbohydrates. Get in a good mix of aerobic & weight lifting exercises.