This thread is a good place to start:
“Whoever said ‘everyone has to start somewhere’ has clearly never tried contributing to an open source project — the Linux Kernel development team in particular is known for its savagery. But if you’re determined to donate your time and talents, there are some things you can do to get off on the right foot. Of course you should pick something you’re interested in and that you use. Check, and double check. You should also research the project, learn about the process for contributing, and do your utmost to avoid asking questions that you can find the answers to. But beyond that there are some hallmarks of beginner-friendly open source projects like Drupal, Python, and LibreOffice — namely, a friendly and active community, training and mentorship programs, and a low barrier to entry.”
Slashdot summary by itwbennett
http://news.slashdot.org/story/13/05/20/1131222/open-source-projects-for-beginners”>Open Source Projects For Beginners
Particularly, a lot of projects need documentation, and some neckbeard programmers aren’t assholes. Read the thread for more on that. But documentation is a great place to start, because a lot of older programmers recommend reading code as well as writing it, and low-level documentation can only be effected by people who can read code. Plus, it’s hard to write about code you’ve written, because you already understand it.
And I wouldn’t be a proper cynic if I didn’t mention that open source looks great on a resume because employers love people who show a tendency to work for free.