The easy way is to change your eating habit & adopt healthy habits.
1) Leave junk food.
2) leave all cola drinks.
3) Take part in games
4) Walk or do exercise. Or you can take part in home work. Increase your physical activity.
5) Never eat anything in between two meals.
6) Never do dieting. You are in growing age, it is not good for you.
7) Take lot of fruit & vegetables.
8) Never skip a meal. Always take food in time.
9) Pray (namaz) is also a physical activity.
Inshallah you will be slim & smart.