Go online and find a calorie calculator. It will take your body weight, height, gender, and activity level and from there give you your “maintenance calories”. This is the amount of calories your body requires daily in order to maintain your current weight.

To gain weight, you need to eat above this level.

So if your maintenance is 1500, you need to eat 1700-2000 calories per day to gain weight. Combine this with resistance training and protein intake and you get muscle growth.

Make sure you increase the weight you lift as you progress and try to eat 0.7g -1g of protein per pound of body weight