You can use repl.it and avoid any setup of Ruby for the first coding challenge. Longer term, you will want to install Ruby on your computer. If you're ready to do that, read on!
Perhaps the most direct source on Ruby Setup is the first chapter of Chris Pine's Learn To Program ebook.
You need a proper code editor; notepad or TextEdit won't cut it. Because we work in pairs, it's important that everyone use the same text editor. Please use Textmate 2 (download here).
Textmate is Mac only; Windows users can use Sublime.
A lot of Ruby development is done at the command line. Mac users can use Terminal.app.
Windows users can run cmd, which is the Windows command line. Run it
by pressing Windows Key-R (launches "run program") and typing cmd
into the box.