A short guide for getting you started into the magical world of OSS.
Are you a musician? Look for musical tools. Are you an artist? Look for some graphics libraries. Find something that works with your other passions to get you going.
If your passion is programming, look for ways to improve the environment.
You earn more selling painkillers than vitamins. Fact of the matter is that people want solutions to painful problems instead of the next new thing they never knew they wanted. Build to solve a problem that you find.
Documentation in OSS is often weak. Being willing to figure out how a library works and commit to its documentation is a good way to get a feel for the project and contribute back some much needed resources to the community.
The other thing that tends to be forgotten to time are tests. Look into the test harness and see if there's anything you can do to improve the tests and get their runtime down.
Look through the issues, see if there are any low hanging fruit and go and see if you can fix that.
This was a quick writeup, I'll fix it up more later but this is in general my response to the question.