To get started, please read https://plone.org/get-started
Our main repo is https://github.com/plone/Products.CMFPlone
There are some relatively easy bugs listed in
For those interested in helping with our documentation, we have wonderful documentation about how to contribute to our documentation. You could read through that page first to get an idea of how we work on our documentation and what you'll need to do to get started.
If you're interested in helping us improve our training guides, an easy way to get started is to read through a guide and let us know in the training issue tracker if you have suggestions on improvements, especially anything that doesn't seem clear or any examples that don't work.
If you're a designer, we'd love to have your help creating or fixing Plone themes. Plone's Diazo theming engine is very powerful and makes it possible to apply arbitrarily rich themes to Plone sites. Find out more how Diazo works, and see recent examples of Diazo themes for Plone.
If you're interested in front end work, we have some ongoing projects with Angular and React that are very active, and a new Pastanaga UI, e.g. https://github.com/plone/plone.restapi-angular https://github.com/plone/plone-react https://github.com/collective/plone-angular-demo https://github.com/plone/plone_client https://github.com/plone/pastanaga-angular
Our core software is in https://github.com/plone and add-ons maintained by the community are in https://github.com/collective