In a pinch, Windows will do, but your best option is probably to find a nice Linux distro and get comfortable with that. It will allow you to follow along with the command line, and when the time comes, it's an easy switch to a Mac.
I'd imagine most of the screenshots in the Rails tutorial will be command line stuff.
I have a Xubuntu VM running in VirtualBox on my Windows 8 laptop that I keep around for coding; it's actually a pretty nice setup. I run the VM on my external monitor, and can use the Windows host to do web browsing, play music, etc. while I'm working - I've even made my Dropbox a shared folder with the VM, so the guest OS can use the files but the host OS handles syncing them so there's no edit conflicts.
If you're running in a VM, one of the lighter Ubuntu variants might be a good choice as well - check out Lubuntu or Xubuntu.
That said, the Linux distro you choose doesn't matter a whole lot, they all tend to be pretty equivalent (well, I'd avoid Gentoo and Arch unless you want to learn a lot more than you'd planned on about building packages from source).