A healthy and regular meal is vital to gain or lose weight. Consuming more calories than you burn will help you in gaining weight. Try to gain weight in a healthy way. Eat lots of fruits and vegetables. Avoid foods like cookies, chips, aerated drinks .You can also do aerobic exercises or exercises which will help you gain muscle mass. The added muscle will increase your metabolism which in turn will lead to increased consumption of calories per day to maintain that weight. You may consult a nutritionist and get a diet chart made which will provide you adequate calories in a healthy way. Avoid taking any supplements as they can be damaging to both your physical and mental health. If you are having a good diet and are free from any illness you should not be worried about your weight.