VR mode in Team Fortress 2, Valve has put together some resources on how to activate VR mode.
Before you boot up Team Fortress 2, hook up your Oculus Rift, add "-vr" to your command line options in Steam and you should be good to go. You can save shortcuts to your desktop with or without the modified command line for quick-launching in VR or vanilla modes.
You can find all of the above and more on Valve's Oculus Rift user guide. The Oculus Rift will have its first dev kits shipped this month; Kickstarter backers should expect theirs around mid-April. Pre-orders are currently open on the Oculus site, though these folks are last in line to receive their goggles sometime near the end of April.