Getting started with JRuby is simple:

1. Ensure you have a recent JDK, version 1.4 or greater (Sun download, Ubuntu instructions)
2. Download the latest JRuby release
3. Unpack it
4. Add the <jruby-dir>/bin directory to your path
5. Away you go!

   > jruby -v ruby 1.8.5 (2007-06-11 rev 3841) [i386-jruby1.0]