The Joomla Tracker has a great roadmap proposed which presents a list of possible features to be included in the Tracker project. The opportunity to volunteer is abundant.
Find an Interest
Look through the list of proposed features or the outstanding issues on GitHub to find the item that you are most interested in and would like to help develop.
Once you have an idea of what you'd like to work on you can create a discussion around the details of the idea (or simply begin writing the code) and request feedback from other users and developers. This process will help to refine the ideas and make sure the finished product works well within the overall Tracker projects goals
Submit Your Code
Whether your fixing an outstanding issue, or creating new code, the final step in the process is to submit your code as a pull request against the Tracker GitHub repository. When submitting your code be sure to be descriptive about the nature and purpose of the pull request and the issues you are addressing.
Once you've done those steps you will see just how easy it is to volunteer and you can start over with the next feature idea!