RugbyD wrote: most important part is to consume 1.5x - 2.0x grams of protien (mostly animal or whey, some soy is fine) per pound of body weight you have, every day. spread it out b/c your body can only process so much at a time. drink tons of water to keep your liver and kidneys from getting pissed off at you.
I would avoid supplements that mess with hormone levels.
To add to what RugbyD and Irish said, protein is the building block of muscle. You also need complex carbs to sustain energy levels during workouts and fats for a healthy nervous system. When I was into this stuff, the recommended ratios were 3:1:2 Carbs:Fats:Protein. Also, eat every 3-4 hours, not sooner. Doing this regularly will turn you body into a nutrient processing machine.
Another tip while I am on a roll: extensive warmup stretching is extremely important, and if you are doing a sport other then weightlifting (esp baseball) do not train with wraps and/or tape. Strengthening the belly of a muscle without its support network is just begging for an injury.
*My knowledge of this stuff is based on personal experience, gym rat lore and the book 'Bodybuilding: A Scientific Approach.' Take it for what it's worth, but it works OK for me.