I am pleased to note that you want to build muscles though being a boy of minor age. You may take up the following tips helping you to achieve in this direction.
1) First of all, avoid junk foods such as Pizza, Burger, Pasta, KFC etc since such foods will disturb your healthy metabolism and cause your body of adding unnecessary fat and cholesterol.
2) Choose healthy diets such as inclusion of Cow- milk, Chickens, Eggs, Dry - fruits in a moderate amount such as Raisins, Walnuts, Almonds. In that way, you will get abundant Protiens apart from healthy omega 3 fatty - acid essential to nourish the body - cells.
3) Engage in brisk walking for a period of 45 minutes both in the morning and evening for a regular basis.
4) You may carry out stretching exercise such as hanging to tone up and stretch your muscles. Better do such exercise under the guidance of an expert so that you follow the right regimen.
5) Engage in Yoga and Meditation so as to achieve peak hormonal balance and indirectly this will help you in building strong muscles.
6) Ensure that your sleep - cycle is not disturbed and you are taking a minimum of eight hours of sleep in night. This will allow your growth hormone to act in a peak way resulting in proper growth of your height and muscles.