The students of the course had the same problem.

We advised the students to look critically at recent open (or just closed) issues and pull requests, as well as e.g., the mailing list for the project at hand. Based on that they could get a feeling what was happening at the moment, and what a typical contribution looks like.

If you want to contribute code, we found that it works best if you work on parts of the code that are currently under active development. This also means that you do not have to have a deep understanding of the full code base: A broad overview suffices.

Some of our students also offered documentation for, e.g., installation or deployment: they struggled themselves, and offered what they learned back to the project.