If you haven't already, Download Perl 6 to prepare.
See the Official Parrot Wiki section on "Contributing to parrot".
The Perl 6 Community Development Server provides free accounts for Perl 6 development.
Besides these links, the Pugs distribution also provides README, READTHEM and READTOO.
A running list of tasks is kept at dev.pugscode.org. It's a catch-all list so something on it is bound to match your skill set!
The precursor TASKS list is in the repo. This document is kept for posterity but is not maintained.
A specific test to help with is Smart Linking.
Rakudo Hacking - Information for hacking and contributing to Rakudo
Patches for updates to Rakudo should be sent to rakudobug@perl.org with [PATCH] in the subject.
You can then see all patches (and others) on Perlbug or just not-yet-applied patches
Preludification Candidates discusses parts of Perl 6 that can be written in Perl 6.
There's also plenty to do related to the test suite, which is written in Perl 6. The most urgent need now is test suite refactoring.
See t/TASKS for other work to do there.
You might also be interested to Start Your Own Perl 6 Module, or work one of the existing ones. (You might also want to check out CPAN6.)
Twenty or so of the 99Problems series remain "unsolved" in perl6. Try coding a perl6 solution, express the solution as a test, and add to t/spec/integration/ in the test suite (see test suite refactoring). Use the existing solutions as guidance. If you have a solution, but do not have a comit bit for the pugs repository, try posting the solution on perl6-users list.
In general, official docs are stored in docs/Perl6 and docs/Pugs in POD format, and they are fair game to improve directly. The exception is most of the documents in docs/Perl6/Spec which is the official Perl 6 specification. Improvements to the official spec should be suggested to the perl6-language list.
See About This Wiki.
Anything reasonable would probably be helpful somewhere.