I wouldnt bother with creatine. It REQUIRES tons and tons of water. Baseball season in the heat+creatine is a recipe for dehydration.
Once baseball season comes around, you'll lose all your water weight.
Take some No2, It gives you an excellent pump in the gym, and there is no cycling required.
Other than that, eat alot of chicken (white), steak, lunch meats, like Dman said, complex carbs. Get your good daily doses of protein. That helps BUILD the muscle after working out. (very crucial)
Another thing i can highly recommend are some good BCAA's (Branched chain amino acids) Take them before your workout, and they are also another building block for working out (along with your proteins)