fucking n00bs.
Go online, and download something called STEAM.
Its like the Itunes of gaming, and they are opening all of the doors for Macs to start good gaming.
Its even better, because they sometimes use cloud computing, so if your machine, cant handle the game, they run it off their hardware and you can play.
You just have to make sure that the game is compatiable with mac, so look for the apple symbol next to the windows symbol.
I know so far you can play Portal, Counter-Strike Source, and Team Fortress 2. Those are ones im 150% sure of, and I know there are more.