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. 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.
One of the quickest ways to give back is to browse our Q&A system for unanswered questions that you can answer. Both questions and answers can be added or edited by any registered user of this website, so feel free to reword questions or add detail to answers that could use more information. We want to limit duplication, so feel free to link new questions to previously answered questions and flag them for removal.
We do our best to stay on top of documentation for the core of Drupal Commerce, but with contributed modules coming out every week, there's still plenty of work to be done. Right now, we've got the core documentation grouped by audience. We welcome feedback and can give you access to add/edit documentation if you request it. 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.