what open source software do you use? think of something you use and would like to improve as opposed to just working on some random open source project.
if your project is small, pick a feature you would like to do, fork it and get it working then email the owner.
if the project you want to work on is big, it will have a chat room, probably on freenode, join the room and the mailing list, ask for a few small bugs to fix (a lost of projects will have these specifically marked) so you get a grasp of the projects workflow, then you can be promoted to working on new things.
if your project is small, pick a feature you would like to do, fork it and get it working then email the owner.
if the project you want to work on is big, it will have a chat room, probably on freenode, join the room and the mailing list, ask for a few small bugs to fix (a lost of projects will have these specifically marked) so you get a grasp of the projects workflow, then you can be promoted to working on new things.