Dear sir it would have been good if u had mentioned height and Weight. Nevertheless here are some tips to help u gain weight.
-have foods rich in proteins like egg ,chicken, sprouted pulses( especially peas,horsegram), peanuts, soybean/soya chunks. These help you build your muscle mass and thus increase your weight in a healthy way.
-do regular aerobic exercises and gyming( it helps your muscle to grow well and even increase your appetite)
-have milk and milk products.
-have regular meals on time.
- you get many protein powders in market. have protein powder mixed with milk thrice everyday.
-have fluids more in form of fruit juices ,tender coconut water, milk shakes, keep yourself well hydrated.