Hi! You have not mentioned your height to kmow what weight you should ideally be. If you are looking to gain weight start weight training exercises under the guidance of a certified trainer. Increase your intake of the following foods-Carbohydrates like rice, bread, chapati, cereal, pasta and fruits; Proteins like fish, chicken, egg white, soya, dairy, lentils and pulses; and good fats like olive oil, fish and nuts. Have a fruit and nut milkshake every evening.