Hi I understand your concern. But for better assessment we need your current weight n height, to give you a proper guidance. For low weight, you need to check your absorption levels n digestion issues if any. Get your hormone levels checked. If all are OK, then you need to have a balanced nutritious meal high calorie but nutritious. Like banana shakes, milk shakes, fruit custard, smoothies, whole cereal like roti, jowar, ragi cereals. Proper exercise to gain weight.