Welcome! We're glad you're here and want to give back.
Drupal Commerce is developed in the open on drupal.org. The code, issue queue and releases all live on the Commerce Core project page. Commerce Core 3.x supports Drupal 10.3 and 11. You can help whether you're a developer, a site builder or just getting started.
Search the Commerce issue queue first. If nobody has reported your problem yet, open a new issue with steps to reproduce it. Testing and reviewing other people's issues helps just as much.
Contributions are made as merge requests through drupal.org's GitLab integration. Open or pick up an issue, create an issue fork, push your changes and open a merge request. Automated tests run on every merge request, and maintainers review them in the issue.
The Drupal Commerce documentation is open to fixes and improvements. Contributed modules document themselves on their own drupal.org project pages.
Commerce is built to be extended. If you've written something others could use, such as a payment gateway, a shipping integration or a promotion type, publish it as a contributed project on drupal.org so the community benefits.
Join the #commerce channel in Drupal Slack to ask questions, coordinate on issues and meet the maintainers.