There is no real central repository for open source contributors. In fact, most help comes from real users with real needs.
To get started, first choose a project that you use and like. Think as a user, what would you do to make it better. What features do you miss?
If you can't find any, go to their bugzilla and search for possible easy fixes to get started... Don't contact their developers before you have some code/patches in hand (lots of people talk about helping, but few do).