Refer to excellent *svnbook.red-bean.com/ (ref. page no. 16, 17 and 183 of pdf if you need to get started quickly) for introduction to (and complete coverage of) svn and *apps.sourceforge.net/trac/sourceforge/wiki/Subversion for sourceforge specific help.
Basically you will first have to create a local repository using svnadmin create. You will then import your existing files into the newly created repository using svn import. Then you will replicate your local repository to sourceforge using svnsync init and svnsync sync. Now you will check out the repository using svn co into a local folder and start working on that. From there on many gui clients will automate the process of committing the code (and other relevant tasks) for you. I am on openSUSE 11.1 and use kde4-svn for that.