As far as getting started, I'd like to see a sticky walking through the steps to create a pull request. But yeah, you can check out from sourceforge using svn, get your changes working, and post the output of svn diff as a patch.
Sticky for creating a pull request: good idea.
JollyGreen: if you know, or are willing to learn, git then working from the git repo ( https://github.com/bitcoin/bitcoin )is smoother. github has built-in support for turning a development branch into a pull request.