I just replied to a similar question that Martin had in another thread, check out my post there, hope it can help!
I struggle with this as well. The best way to contribute is to catch bugs, since Rails 2.0 was about improving the core and not adding a ton of new features. You could simply browse through the thousands of lines of Rails source code, which subsequently would also teach you to master Ruby if you haven't already.
Alternatively, you could always write plugins and other addons to Rails to enhance functionality. This is a fun way to give back to the community.
-Kevin Elliott "WeLikeFood Beta":http://www.welikefood.com Discover great restaurants. Discover great food enthusiasts with tastes like yours.
A good place to start is the "dev.rubyonrails.com":http://dev.rubyonrails.com - here's you'll find some (brief) instructions on how to get started.
Also, I can heartily recommend the railscast screencast on contributing to rails: "http://railscasts.com/episodes/50":http://railscasts.com/episodes/50 - this podcast takes you through step by step on how to create a patch for rails.
Josh Susser (of "has_many :through":http://blog.hasmanythrough.com has put together a nice little presentation on the topic. Check it out "here":http://blog.hasmanythrough.com/2007/5/24/laying-tracks-slides.