Good open source projects should have documentation for you to help you get started. Like the other posts, there are many ways you can contribute, and every little bit does help.
As far as I know, no one will be standing over you expecting you to finish anything, unless you made some kind of agreement to start.
I would suggest finding something you enjoy and just start. When you are ready to submit your changes be ready for great feedback and a ton of learning. You will be working with some really passionate, skilled guys so there will undoubtedly be a big, but rewarding, learning curve.
Our project is .Net based, so not really a fit for your skills, but feel free to check it out or share with your friends: https://github.com/Warewolf-ESB/Warewolf-ESB
Good luck, and most of all, have fun!