The Cassandra documentation is maintained in the Cassandra source repository along with the Cassandra code base. To submit changes to the documentation, follow the standard process for submitting a patch (Contributing Code Changes).
Subscribe to the user list, look out for some questions you know the answer to and reply with an answer. Simple as that! See the community page for details on how to subscribe to the mailing list.
Reviewing patches is not the sole domain of committers, if others have reviewed a patch it can reduce the load on the committers allowing them to write more great features or review more patches. Follow the instructions in _development_how_to_review or create a build with the patch and test it with your own workload. Add a comment to the JIRA ticket to let others know what you have done and the results of your work. (For example, “I tested this performance enhacement on our application’s standard production load test and found a 3% improvement.”)
Often, the hardest work in fixing a bug is reproducing it. Even if you don’t have the knowledge to produce a fix, figuring out a way to reliable reproduce an issues can be a massive contribution to getting a bug fixed. Document your method of reproduction in a JIRA comment or, better yet, produce an automated test that reproduces the issue and attach it to the ticket. If you go as far as producing a fix, follow the process for submitting a patch (Contributing Code Changes).
Test coverage in Cassandra is improving but, as with most code bases, it could benefit from more automated test coverage. Before starting work in an area, consider reviewing and enhancing the existing test coverage. This will both improve your knowledge of the code before you start on an enhancement and reduce the chances of your change in introducing new issues. See Testing and Contributing Code Changes for more detail.