Should I just go to SourceForge and try to find something that looks interesting by doing a search? Or is there a Ruby-specific website that helps you to find open source projects to contribute to?
After you find it one, do you usually just send an email to the project owner to see if you can help or do you just start submitting code to their repository and they will take it or leave it after reviewing it for quality?
I'd recommend going onto github to look for projects. You can search around, check things out easily and its easier for the project owner to manager your changes.
The standard hosting site for Ruby projects is RubyForge. Another site that is slightly older than RubyForge, is the Ruby Application Archive (RAA). In addition to RubyForge, there are also a lot of Ruby projects hosted on SourceForge. Projects that are specific or related to JRuby, are sometimes hosted on CodeHaus or Sun's new project hosting site Kenai. For IronRuby, some projects live on CodePlex. Projects that use the Git Version Control System, are often hosted on either Gitorious or GitHub, whereas projects that use the Darcs Version Control System tend to be self-hosted.
However, especially larger Ruby projects often have their own infrastructure, e.g. Merb, DataMapper and of course Ruby on Rails.