My advice would be to find a project you use enough to be familiar with what it does and vaguely how it works, and try to help.
You could start out by trying to reproduce reported bugs/issues. Report your results back to the issue tracker.
Then, (if you reproduced the issue) if you're able to, try to track down the cause of the issue. If you find the source of the issue, you can either report it to the maintainers (i.e. if its a language you're not comfortable enough to fix it yourself) or, if you can fix it yourself, (and TEST it after!) and then either send a patch, create a pull request, etc.