Building muscle mass is a simple science. You train your muscles and create micro-tears in the muscle fibres, you then feed your body with excess quality calories and amino acids, combined with good sleep, and your body will repair the micro tears, building bigger muscles to adapt to the physical stress. Although this is a simple process, it can be hard to achieve, as your body requires a lot of calories from the appropriate macro nutrients to ensure your body is consistently in an anabolic state.
To help this process become a little easier you can use nutritional supplements to ensure you are feeding your body the correct nutrients to achieve maximal muscle repair and growth. By using a protein powder or mass gainer, you will be feeding your muscles the amino acids they require to initiate the repair process, along with the complex carbohydrates to supply your body with an energy source to replenish you depleted glycogen stores after training. You can also use other supplements such as creatine to build your strength, which in turn builds bigger muscles, along with testosterone boosters.