You should not go to Gym at
your age. Teenage is the best time for sports, if possible you can join swimming. If not you try doing skipping, jogging in the neighbourhood, floor exercises, yoga, basketball etc.
Calculate your BMI and also according to your age prepare a neat diet chart and stick to that. Do not consume fried food, desserts, choclates, icecreams, fast foods, etc.
Exercise combined with Diet will do wonders on you. Take care!