First of all, thanks! It's really awesome that you're interested in contributing to Rocket Chat. If this is the first Open Source project you will contribute to, it'd be cool if you read GitHub's excellent guide "Contributing to Open Source".
You can find Rocket.Chat repositories here. If you see some issue you are willing to work on, just comment on it. We will add an In Progress label, and that's it! Want a simple task to get you started? We maintain a list of those! Looking for other developer guides? We have it here in the documentation!
For contributions to code or documentation, you can fork the material, apply your contribution, and submit a Pull Request.
Please consider donating.