Increase your calorie intake, increase the amount of meals you consume, and increase your consumption in carbohydrates. Make sure that you're not consuming too much saturated fat and also make sure you watch out for your sodium intake (you don't want your body to retain too much water weight and make you look very bloated). 

figure out the number of your calorie intake. You can increase the number of calorie intake subsequently. Oily food, animal fat, nuts are some high calorie food.

Another thing is to have 2 bananans and a glass of milk everyday in the morning. This sure short tip. Just give it a try.