[myffical]

Start with projects that you personally use and enjoy! You'll have more insight in the project, and you'll feel more motivated. (e.g."I'll write a patch for feature X becuause I want to see it in the next release.")

Look for projects that are in the middle stages of maturity. A project that is too mature will require technically difficult work; one too young will require lots of discussion on design and specification.

Work on easy tasks first, such as minor bugs or gaps in documentation. These tasks tend to finish faster, and they'll encourage you to go on to tougher stuff.

Get familiar with the tracker (e.g. Trac, Bugzilla) and repository (e.g. SVN, Git, Mercurial). In particular, know how to create and submit a patch. Most projects rely on these tools for collaboration, so you should cater to their workflow processes.

Good luck!