One trick is to star fixing typos. It's short and they will probably get merged, and you will learn what are the informal rules of the repository. (Do you have to sign your commit? Write a nice message? Spaces vs tabs? Add a test case in case you change the code?)
Also, don't underestimate how helpful is to fill a nice bug report with foolproof reproduction steps. And in case someone else fix the bug, it's good to take a look and try to understand the code, so next time perhaps you can fix a similar bug.
