Eat a Lot of Protein.
Protein is essential when it comes to building muscle and can be found in chicken, fish, eggs, milk, dairy products and some nuts and vegetables. Most experts recommend at least 1 gram of protein per pound of body weight for the best results. If consuming enough protein each day becomes difficult, add a protein shake as well.

Be in a Caloric Surplus.
In order to build muscle you need to eat more calories than you burn. Use a calorie calculator to find out your BMR (Basal Metabolic Rate) and adjust that according to how active you are. If you have a sedentary job you will need fewer calories and if you are very active you will need more.

Eat More Often.
Have 4-5 meals per day and eat every 3-4 hours. Avoid being hungry!