You need to determine how many calories you need to intake in order to begin putting on weight. Generally. to see the best results and gain weight in a healthy way, you are going to want to add about 500 calories daily to your diet. If you will be working out or practicing any heavy physical activity for more than 30 minutes a day, you are going to want to add even more calories. You should determine how many calories your activity burns and add those calories on to your daily intake, as well to ensure that you continue to gain weight instead of lose it.