You may want to check out OpenHatch, a project that, among other things, organizes open-source projects you can help with by difficulty and language. This should help a lot with the learning curve. There are also some "missions" for beginners that teach development skills like how to use version control or apply a patch.

Edit: Huh, looks like all the available tasks are "bitesize" difficulty right now. I don't recall that being the case before.