Bootcamp is the Apple-provided set of tools and drivers that allow you to install the full version of Windows on a portion of your hard drive, and run it natively as if you were using any ordinary IBM x86 PC. It's not an operating system on its own, just something to help you have Windows on dual-boot on your Mac. You need a copy of Windows with a license key. Run the Bootcamp Installer from the Launchpad and it'll take you through the process. Or check Apple's support pages for more information.
After it's done, you load up Windows by opening the Startup Disk app in Mac OS, or holding down the Alt (Option) key after you turn your Mac on, and it'll give you a choice between the two operating systems. Then you'd need to install Steam and Team Fortress 2.
If you're clever, you should be able to copy your TF2 installation off Mac OS and use it as a foundation for your Windows install so you don't have to download TF2 completely. You'd need an external hard drive or something since Windows can't access the Mac partition.