Hi
Both Gaining and losing weight is not easy...but with consistent efforts with good diet and healthy lifestyle both put together will slowly help you in achieving your target..
For gaining weight consume high calorie diet consisting of moderate carbs, protein, fats, EFA'S, all variety nuts, seeds, dry fruits , fresh dates, red and Kerala variety bananas, home made healthy sweet items, sweet potato kheer, full cream milk and it's products, small frequent meals, chew food well, drink enough water, fruit juices, enjoy your food thoroughly with every morsel, ghee in dal and light exercises..are all helpful...with daily consistent efforts in place..
Next Steps
please consult doctor for any medical issues if any and know the underlying cause for underweight..