The secret is adjusting your "energy equation."
A pound of weight equals about 3500 calories. [If you are underweight you need take in more calories. If you are overweight you need to burn more.] If you wanna maintain your weight, you should burn as many calories as you take in.
Th MyPyramid suggest that teens get 60 minutes of physical activity most days. Calories vary depending on your age.
Females; 14 - 18 : Approximately 1800 to 2400 per day.
Males; 14 - 18 : Approximately 2200 to 3200 per day.
Exercise or other physicl activities are a good way to burn calories.