Muscle is primarily made up of protein. To increase muscle size, you must perform regular resistance training. Nutrition plays a huge role in this development as well. There are four main components that influence muscle gain.
1) Hitting your daily protein targets provides your body with the building blocks for muscle gain. For individuals trying to build muscle, it is recommended you consume 1.8-2.2g/kg of body weight. Consuming this level of protein will increase muscle protein synthesis to promote a positive muscle protein balance.
2) Consume a consistent amount of protein at each meal. Around 0.4-0.6g/kg protein at each meal will stimulate muscle protein synthesis. An easier way to determine your protein needs at each meal is to calculate your daily needs and divide by the number of meals you plan on eating. As long as that number is at least 20g, you are good to go.
3) Eat high quality protein. A complete protein contains all the essential amino acids. If you cannot consume animal protein, make sure you are pairing 2 incomplete proteins together.
4) Eat in a 5-10% caloric surplus. You cannot build muscle without consuming enough calories.