If you haven't already, install RVM. If you don't plan on compiling the MRI interpreter, you may be able to skip installing Xcode, which is installed primarily for GCC. You won't be needing GCC with jRuby.
If you have already installed RVM, it's a good idea to update it. While it's not necessary, it's usually a good idea. The upgrade process is painless, run rvm get latest and then rvm reload.