1g of Protein=4 calories

Proteins build muscles. That's almost all you need to know. The best kind of proteins to get is from animal protein because you get all your amino acids. We suggest you get about 1 gram of protein per body pound per day if you want to gain muscles at a good rate. OR 1.5 grams of protein if you want to bulk or gain as much muscles as possible.
Here's an example, let's say you're 170lbs, you should get 170 grams of protein per day (by following the first principle).

 

Here are some good Protein source.
 

Fish
Lean beef

Chicken
Turkey
Greek yougourt
Whole eggs

Eggs white
Cottage cheese

Almonds

Quinoa