I can see three or four possible approaches:
1. Pick a project at (more or less) random; something that "scratches an itch" for you personally. It may or may not turn out to be maintained, the maintainer(s) may or may not be receptive to pull requests, etc., but nonetheless, you can grab it, fork it, and start hacking. If it's a project with a mailing list / forums, you might want to lurk a bit first, get a feel for the community, and maybe even post a message saying "Hey, I was thinking about adding Feature X, or fixing Bug Y, any interest in that?"
2. Ask around for a project that's actively seeking more help; maybe even one that's being run by a fellow HN'er. Build a good relationship with said HN'er and maybe get them to act as a bit of a mentor and walk you through the early phases of your involvement.
3. Check http://www.reddit.com/r/FOSSHelpWanted/ and find something there.
4. Start your own project from scratch and put it up on Github.
And for a slightly self-serving bit... if you know (or want to know) Groovy and are looking for a project like mentioned in [2] above - that could use some help - I might just know of one. Shoot me an email if you'd like to hear more. :-)
