There are actually two ways to do this. One is through the use of import which doesn’t put your active code base under version control. The other method is to just create an empty project in the repository and then check it out directly into the code base. Then just do an add and an a commit:
svn mkdir <repo>/newProject
svn checkout <repo>/newProject /path/to/codebase
svn add /path/to/codebase/*
svn commit /path/to/codebase -m "adding initial codebase"
There is a good tutorial on how to create a new project here: http://web.archive.org/web/20110316170621/http://www.duchnik.com/tutorials/vc/svn-command-reference