I was just looking into this subject today, right before I read this post. Here's what I'm doing and would recommend.
Join the forum related to your Linux/BSD distro of choice and check out the project page and see what needs help and what needs work. See if there's anything you're interested in already and go with that. If not, a good project to join would be bug tracking; for an example see the corresponding project from Fedora:
Follow the directions, get your feet wet and eventually, if not from the very start, ask for a mentor already in the project and ask questions of them. Follow the project #IRC for a while and you'll get a good feel for where the work is needed. If you graduate into more technically challenging projects while at the same time being beneficial to the project, you will surely learn a lot and hopefully gain some invaluable experience.
Join the forum related to your Linux/BSD distro of choice and check out the project page and see what needs help and what needs work. See if there's anything you're interested in already and go with that. If not, a good project to join would be bug tracking; for an example see the corresponding project from Fedora:
https://fedoraproject.org/wiki/BugZappers
Or Debian:
http://www.debian.org/Bugs/
Follow the directions, get your feet wet and eventually, if not from the very start, ask for a mentor already in the project and ask questions of them. Follow the project #IRC for a while and you'll get a good feel for where the work is needed. If you graduate into more technically challenging projects while at the same time being beneficial to the project, you will surely learn a lot and hopefully gain some invaluable experience.