You'll probably want to install the MRI Ruby interpreter (the official Ruby interpreter, the one you're probably already familiar with). To do that (after you installed the build dependencies, see the previous steps), it's a simple rvm install 1.9.3. This will give you the MRI interpreter version 1.9.3 (the stable release at the time this article was written) at the latest patch level.
$ rvm install 1.9.3
And that's it. Remember to rvm use 1.9.3 before you start using Ruby and that's it, Ruby is installed.