You can gain weight by eating unhealthy by eating food such as junk but that will make you unhealthy and will give you the wrong form of weight gain. Do the following lifestyle changes for a healthy gain of weight and height: - Eat healthy, avoid junk food, Increase your calorie intake, eat more calories than your body burns in order to gain weight. Aim for 300-500 calories per day above your maintenance level - Eat increased amount of protein as protein will help in muscle gain - In order to gain weight, eat at least 3 meals per day and make sure to eat plenty of fat, carbs and protein in each meal. - Eat food like nuts, dried fruit, high-fat dairy, Fats and Oils, grains, meat, tubers, dark chocolate, avocados, peanut butter, coconut milk, granola, trail mixes. - Exercise regularly