First think to consider is what you're interested in, there are many places to look at, like github.com, sourceforge.com (lot of orphaned projects), openhatch.org, google code of course and you should also check http://open-advice.org, is somewhat different from the others:

"Open Advice is a knowledge collection from a wide variety of Free Software projects. It answers the question what 42 prominent contributors would have liked to know when they started so you can get a head-start no matter how and where you contribute. "

Good luck!