You should eat good food but not junk food. Do not advise dieting. Because you are in growing age you need good food to grow healthy.
What you do increase your physical activity.
Join game club. Take part in sports.
Walk or run daily for 30 minutes.
Do house work to help your mom.
Do not take chocolate, chips & drinks. Drink plenty of simple water.
Inshallah you will become smart.