There are three basic things to be accomplished to build muscle. First, strength train to break down old muscle fibers and encourage your body to replace the old one with even more new muscle fibers. Second, eat enough calories to discourage your body from using your muscle fibers for energy. Last but not the least, eat enough protein to provide your body with the building blocks it needs to repair old muscle fibers ad build new muscle fibers.
The best sources of protein include:
Whey – One of the two major proteins found in the milk of humans and other mammals, such as cows. Nature selected it as one of the best proteins to help babies grow. It’s the perfect post-workout protein because whey is digested and used quickly. It has all the amino acids you need.
Casein – The other milk protein. Unlike whey, casein is digested and used slowly, making it a decent post workout protein, but an excellent before bed protein.
Egg Whites – Egg yolks are good for the body when eaten in moderation, but egg whites are far better. Egg whites have all the amino acids you need and can be fairly inexpensive if you buy whole eggs and separate them yourself.

Skinless Chicken Breast – For those who just want meat. Skinless chicken breasts are extremely lean especially if you remove all visible fat. They are muscles themselves, so they obviously have the amino acids you need to build muscle.