For a preteen take up a sport like soccer or lacrosse. Can't take any diet pills cause ur body still growing. More healthier ways instead of coffee (if u drink coffee) try green tea with honey. Replace water with soda or juice.
Eat foods from the basic food groups. Decrease fried foods, fast foods, junk food and NEVER skip a meal. Skipping meals will slow down your metabolism. Eat something within 30 minutes of waking up to fuel your body.