Protein is of course a must, but don't forget about good carbohydrates. Your body needs carbohydrates to transport amino acids to the muscles. So if you're eating tons of protein, and no carbohydrates, you're going to have a hard time building muscle. And of course, don't forget about including good fat in your diet. My diet consists of 50% carbs 30% protein and 20% fat. My favorite and biggest meal of the day is breakfast. Usually I'll have something like 1/2 cup of oatmeal mixed with 1 cup of egg whites and 2 tbsp of ground flaxseed. I'll eat maybe a banana and a cup of Kashi cerial with it. That example meal is about 645 calories consisting of 44g protein, 95g carbs, 12g fat. The micronutrition breaks down at about a 59/28/13 ration of carb/pro/fat. I consume lots of carbs in the morning and then after my workout. Of course, how many calories you need will depend on your body composition. But here are some of my favorite foods:
Pro:
Egg whites
Chicken breasts
Turkey breasts
Salmon
Water-packed tuna
Whey protein
Myoplex
Various protein shakes
Carbs:
Oatmeal
Fruits and vegetables
Rice cakes
Brown rice
Cliff bars
Yogurt
Fat:
Almonds
Peanut butter
Ground flaxseed
. . . and so on. Good luck!