Hi, Weight gain involves a balanced diet and physical activity.Eat calorie dense foods like nuts,egg whites,lean meat,fish etc.Include plenty of fruits and vegetables as soups salads etc as a snack.Eat 5-6 times a day and drink 6-8 glasses of water.Sleep for 8hrs in the night.It helps in building muscle mass.Good protein aid in building lean muscle mass.Avoid heavy fried foods as they only add fats are not a healthy weight gain option.Include good fats like omega 3 in your diet.