A great place to start would be writing failing tests for very simple functionality. The tests that are in the parrot repo are very minimal. An example would be addition tests such as put 1+2. Basic tests are preferred at this point. But if you write a failing tests, even a complicated one, I'll attempt to implement the functionality so that it passes.
The core classes such as Fixnum, String, Array and Hash will eventually be implemented in C as PMC's for speed reasons. PMCs are a little complicated, but if your interested they are another much needed feature I just haven't gotten around to. While unused stubs for Ruby PMCs exist in the subversion repository.
Jump right in, ask a billion questions. I'm always on #parrot at irc.perl.org. Patches can be sent to me at cardinal@tewk.com.
Thanks again for volunteering.
Kevin Tew