First of all you should close Xcode, locate your project in the Finder, and create a directory hierarchy appropriated for SVN
/projectdir
/projectdir/branches
/projectdir/tags
/projectdir/trunk
Then, you should put your Xcode project directory into trunk.
Once you've done that, and created your repository on your server using svnadmin create PATH-TO-PROJECTDIR, you can import your local copy into the newly created repository on the server. Note that svnadmin doesn't create intermediate directories contained in PATH-TO-PROJECTDIR.
Use svn import projectdir svn+ssh://your-server-url/your-repository-path -m "First commit".
Now you are able to check out your trunk (with Xcode or svn command) and version control in Xcode will be enabled.