Check out this nice set of instructions for getting Ruby on Rails set up on Mac OSX.
The Hivelogic Narrative: Articles: Building Ruby, Rails, LightTPD, and MySQL on Tiger
Building Ruby, Rails, LightTPD, and MySQL on TigerTechnology | Web Design | Development | Mac OS X | Ruby on Rails | 12.01.05 – 09:22 PM
What follows are updated instructions for manually building and installing Ruby, Ruby on Rails, MySQL, and LightTPD on Mac OS X 10.4 (Tiger).