Because a project is open sourced doesn't necessarily mean that you can contribute to it or that direct contributions are accepted. You should join an open source project that your personally interested in and one that has topical guides on how you can contribute to the project.
For your first project, you should definitely pick one that has a very accessible community, i.e. even the most inexperienced or humble contributions are welcomed, and one that documents how the development process is organized and executed, one that can fully address many of the questions you probably have at this point about what it means to take a ride on the track of open, collaborative community development:
- How are contributors organized and what are their roles?
- How is the development process planned?
- Are there different kinds of releases that the project publishes?
- What kind of changes are allowed in a certain release?
- How, when and where can I propose or submit certain code changes?
Until you familiarize yourself with the overall design and how different components communicate with or affect other parts of the system, you should generally avoid tackling those bits. Each project has certain areas that are easy to get involved in, such as improving the documentation and the translations and then there are also certain portions of the project's code that may fall directly into your area of expertise, say cryptography, where you can immediately provide improvements.