You've picked an OS!
As you don't know what Linux is, it's preferable to skip over to this section.
Oh, I see how it is. Windows only supports x86 CPU architectures (hence the popularity of it today), so if you want to port Windows onto ARM, Windows RT won't work, so consider editing ReactOS and make it ARM compatible. (If you know what to do, that is. :) It would be awesome to the community!)
But do understand that even if ReactOS itself would work, you could run none of the x86 Windows applications on it.
Hello, newcomer (to Linux and/or the Raspberry Pi). To get Raspbian, the official OS for the Raspberry Pi, click here. Raspbian is a specialized version of Linux just for the Raspberry Pi!
Fedora or Red Hat
Shoot! Red Hat is not making an ARM version available! Please try the default distro.
Hmm, congrats! Arch Linux IS supported by the RasPi Foundation! Please click here for an Arch Linux download for the RasPi.
Ok. Good for you. Please click here for an appropriate Debian download for the RasPi.