I would recommend in addition to contributing to other projects, to try to start something of your own. Find something small and useful you can work on, and release it. It doesn't matter if it's small - the experience you get maintaining your own project will teach you a lot about how to contribute to other people's projects.

My first open source project was a PHP script called "DBMailer" that I wrote when I had been programming for only a few weeks. I'm sometimes embarrassed that it's still floating around the internet with my name on it, but I learned a lot from the feedback I got from it at the time.

I wrote a blog post a while back with some do's and don't's for both contributors and maintainers, maybe you'll find it interesting:

http://njclarke.com/posts/how-to-do-open-source.html

Good luck, and stay enthusiastic!