Irrespective of Bogdan's comment, @Rob's suggestion is a good one for a couple of reasons.
If you pick an open source project that is in an area that interests you, e.g. if you're into music then working on the Audacity project, then:
- you will be more motivated because the project is trying to solve aspects of a problem space that you have an interest in
- you bring your particular knowledge to the project which will be useful to the project
- you will (probably) learn more about your area of interest from others involved in the project
- you finish up with an improved version of the project to use
HTH
cheers