Windows requires 20-30GB of space free on you startup disk (Bootcamp assistant takes care of partitioning this).
All you need is an iso of Windows, which you can either get by buying a copy of Windows (7 is about $50) or download the technical preview of Windows 10 for free as I did (there are less legal methods of getting it for free but Windows are giving the technical preview of 10 away until the full release so I would recommend getting that). I recently set up Bootcamp on my MBP and it's really easy to do, just follow the instructions on Apple's website or go to Boot Camp Assistant on your Mac. BootCamp is a great method since it runs Windows as a completely different OS from OS X, meaning you get all the RAM going to your game, rather than sharing it between OS X and Windows, if you use Parallels or PlayOnMac.