Welcome! Yes, we really love having you here, looking to give back. : )
Drupal Commerce is a microcosm of the Drupal project, with its code and issue tracker hosted on the Drupal community project website and documented on our sub-domain docs.drupalcommerce.org. You can contribute to the project in any number of ways, whether you're a Drupal developer, Drupal site builder extraordinaire, or a humble newcomer. And we're proud of our contributors, so much so that we're highlighting many of them in our Contributor Spotlight
Office hours
Want to get involved with Drupal Commerce? We host two office hours where contributors and maintainers can work together.
Documentation
Documentation for Commerce 2.x has been created in a repo on github.com and hosted on docs.drupalcommerce.org. We welcome feedback and ask that you please use github to open issues and we'll do our best to create and maintain the best documentation for Drupal Commerce around.
For contributed modules, most of them have their own pages listed on drupal.org and link from their project pages.
Modules and Themes
Drupal Commerce is a framework designed to be modified, twisted, and tweaked using a very strong and obvious API. Most days and nights you will find like-minded developers working through hooks_() and pounding out custom themes for their clients in IRC #drupal-commerce on freenode.net. Writing a module that does what you want is simple enough, but we ask that you contribute your code back at least as a sandbox or a patch in an issue que so others can benefit from your amazing module.