Gaining muscles requires you to eat certain foods that help you grow
your muscles as well as build up exercises:

Muscle is protein, so if you must gain muscle, then you will need to
eat plenty of proteins.

Eat foods with adequate calories, you need more calories to grow your muscle.
Eat lots of good fats, good fats are important for growth.

You will need more than three meals daily since you require at least
4000 calories daily to build your muscle, you need to eat more, maybe
small portions of food with adequate calories five to six times a day
so you'll also have constant supply of energy.

Carbohydrate is important in the body, eat lots of carbohydrate in
order to have good supply of energy for build up exercises.

Take in plenty of water.

Have a meal before workout, this is important in keeping up your
energy level while you exercise. A meal of carbohydrates and protein
would be ideal.

A good meal after workout should be taken to replace lost energy and calories.

Have as much rest as you can, rest is good to recover from workout stress.

Engage in regular muscle build up exercises.