File a JIRA to submit a bug report.
If you need access to edit the wiki, make sure you've created an account and send an email to firstname.lastname@example.org with your userid and a description of what you're interested in working on and someone will grant you access. Same goes for JIRA if you need to be able to assign or resolve issues.
Please visit our Becoming a committer page for details on how to become a committer.
If you are a new contributor, the best starting point is probably one of the tickets marked with the starter label. Most of these should be fairly self contained and approachable.
If you are interested in taking your contribution a level up you might instead want to look at tickets marked with the starter++ label.
Another great way to start contributing is adding examples. This might help you deepen your own understanding of Geode while also helping others.
If you are already well familiar with Geode and want to contribute a larger set of features the best place to start is with a discussion on the dev list.
Feel free to contribute to this wiki – make things easier to understand or just share your experience with Geode.