First off you have to understand what creatine is and how it is benefitted.
Our bodies produce it in small qty's. It is also found in red meat like steak, but you have to eat it pretty rare to get any.
Creatine hydrates the muscle cells which in turn inflates them with water, which in turn causes better protein synthesis (the break down and absorbsion of proteins.
To get more creatine into your body, you need to supplement with it and use a very high glycemic transport which causes an insulin spike which forces the creatine into the muscle. I currently use one by Twinlab called "Creatine Nitrate Fuel", another good one is "Cell Mass" by BSN. Taking creatine by its self will do nothing, it needs sugars to drive it home.
I have had mixed results with creatine, sometimes it works, sometimes it doesn't. You need about 20 grams a day, in 5gr doses throughout the day.
If i were to limit myself to one supplemnt like that, it would be L-Glutamine. It is the most abundant amino acid in our bodies and by adding more, it keeps you in a positive nitrogen balance and anabolic, but both glutamine & creatine can be used together for a double blast.
I can assure you one thing.....if your not getting enough calories in your day, NOTHING will give you the results your looking for.
Like stinky mentioned, 1 1/2 grams of protein MINIMUM per pound of body weight for positive muscle growth, 2 if your taking hard core supplements.
One thing i will disagree with on what stinky said was an Isopure drink for post workout. There are no carbs in that drink and carbs are more important for post training than even protein. Glycogen is the reason for that. Carbs replentish the glycogen stores you just forced out of your muscles with intense training, you have GOT to drink a high glycemic carb immediately following your workout to get those glycogen levels back up quickly. The Creatine Nitrate Fuel or Cell Mass will do a great job at accomplishing that. I am also a fan of "Aftershock" by BSN, excellent post workout drink.
Don't hate me Stinky!! ;-)