You change your life style which will help you to gain weight.Eat more food more frequently.Drink whole cream milk or milk shake 300 ml/day.Take banana 3-4pcs/day.Use weight gainer nutritional supplement.Add cream,butter,cheese,jam,jelly,ghee with foods extra.Take cake,pastries,dark chocolate,ice cream.Eat more fish,egg,meat,soy milk,soybean,soy product,cottage cheese,mushroom,lentil,yogurt.Use extra healthy fat in cooking.No water just before or during meals.Avoid smoking,alcohol.Drink lots of water.Do exercise like weight lifting and muscle building exercise which will help you to gain weight.Take proper rest.