What to do if i'm a 17years old boy.but my weight is 39kg. i'm too much thin.so i want to increase my body?

What's your BMI?: If your height and weight are proportional- according to your BMI- then this may be the appropriate weight for your body. If not, I would recommend a healthy diet with low fat proteins, whole grains, and plenty of fruits and veggies. Exercise will build muscle mass (which weighs more than fat) and keep you healthy. It will still take time, so don't expect overnight results!!