I think the first step is to decide what you're interested in learning and reaching out to a project closely aligned with your goals. It is possible you will be rejected, but still worth making the initial effort to find one that would hold your interest. See the following blog post (and comments) explaining the ideal mindset needed when going into this: http://www.hanselman.com/blog/OpenSourceIsAThanklessJobWeDoI...
The specific group that I had in mind was Python; they have implemented a mentorship program at http://pythonmentors.com -- I'd recommend starting there and asking for pointers to other Python projects. With resources like http://learnpythonthehardway.org/ available you should be able to hit the ground running.