Also known as bulking, you simply gain weight by eating more calories than your body uses throughout the day. You may have to experiment a little to find the right number of calories that you need to eat, but try about 3000 - 3500 for the first week and see if you gain any weight. Everyone is different with how much food they need to eat, so experiment to find and see what works for you. While bulking, you need to make sure that you aren’t just adding fat. Track your progress in the mirror or in progress pictures to ensure that you are bulking properly. Some fat gain is inevitable while bulking, but don’t get carried away with it. I see way to many people posting year apart progress pictures of them just gaining fat and little to no muscle – do not be one of those people. This is easily avoided by having a good diet and routine.