To Build Body Weight, you have to Increase your daily caloric intake by 1,000 calories so that your body will have the energy to devote to building muscle tissue. Besides, introduce more protein into your diet to supply your muscles with the nutrients they need to increase their mass. In addition, start a regular weight-training routine exercising three to five days a week to begin building muscle. Other than that, sleep regularly and for longer periods so that your body has enough rest time to repair itself and build more muscle mass.