Bootcamp is pretty simple to set up, but you will need a Windows install of some sort.
If you go to Applications -> Utilities -> Bootcamp Assistant or something like that, it’s just a step by step process to set up a partition on your hard drive. It’s really self explanatory, just follow the directions and set aside however much space you want for your partition (30GB is plenty of space if you really just want to play GGPO or whatever).
Then you have to get a copy of Windows XP/Vista/whatever. I think you can still beta test 7 for free, but I don’t know if GGPO works for that. Anyway, buy a copy or burn a legally acquired iso to a disc (and I say that not only so I don’t get banhammered, but also some shit fucks up if your Windows install isn’t legit). Boot up your computer to the new partition by holding ALT on startup to get to the boot menu, and put in your install disc. Follow the directions there to install Windows.