Look for places the code might not currently be clear, and try to improve clarity without changing behavior.
Once you've done that kind of thing, sky's the limit as far as I am concerned. I'm working on understanding and improving the parameter parsing behavior at the moment.
That's one of the only things I'm frustrated about - I wish there were a clearer list so that I didn't have to troll for tasks that need doing, or ask a core member on IRC/campfire
One of the most common places where that's true is in the ActiveRecord issues, which are about half of the open ones: https://github.com/rails/rails/issues?direction=asc&labe...
Even just making a little Rails app or script that re-produces the bug would be helpful. Writing a test case that's failing would be mega helpful. Fixing the bug would be incredibly helpful. :)
If there is anything I can do to make this more clear, please let me know. Documentation is something I enjoy.
Once you have that installed, you basically don't have to worry about dependencies that you need to bootstrap the Rails test suite.