What version of Windows do I need to download for me to run Bootcamp on my Mac?

Hello Everyone:

I have a MacBook Air, OS X Yosemite, version 10.10.5. I would like to run a Windows-based CD on my Mac. I have the superdrive, but am not sure which version of Windows I should download before installing BootCamp. Can anybody walk me through this. I'm NERVOUS!!

Rae