- Contributing to the Collective: Plone is more then the core, one of the things that makes it great is the large number of add-ons freely available and it's something you can contribute to. Whether its adding your own product or patching somebody else's, it's a good way to get more experience with the project and get to know the key players in the Plone community. As always, you will find plenty of help on #plone irc, the mailing lists and the extensive documentation on plone.org. For more information please see http://collective.github.com.
- Contributing to Plone Core: The easiest way to contribute code to Plone Core is to participate in bug fixes. For a list of currently open bugs, please check out our trac ticket reports. Once you are familiar with fixing tickets, please consider contributing new features and major architecture changes to Plone by participating in the PLIP Process. All the information you need and more on how to contribute to core lives at http://readthedocs.org/docs/buildoutcoredev/en/latest/agreement.html.
If you are just getting started or still ramping up your code ninja skills, consider helping out on one of the many teams in your area of interest. There is nothing wrong with lurking for a while until you feel comfortable contributing. Most of these teams will have tasks that segue nicely into learning core development.
When in doubt, feel free to hop into #plone and ask what needs to be done. There is more than enough to go around!