To weight gain you should consume energy and protein rich food and do some physical exercise. Make your diet plan. Take heavy breakfast in the morning which includes wheat bread or chappatis (3 to 4), fruits like one banana or apple or any seasonal food, with a glass of Milk or cup of coffee. Remember to include cereals like wheat, rice, jowar, bajra and pulses like soyabean, bengal gram, green gram in diet. Consume milk, ghee or butter. Eat spruoted beans. Include eggs, fish and chicken/meat in your diet. Regular 10 minutes jogging, 15-20 minutes aerobic exercise will help in muscle building and prevents fat deposition. Do not drink carbonated drinks, junk food or fast food. Hope this will help you.