If you have the need to create a tag from your current source code, you can use the copy command from the shell of your preference:
$ svn copy http://svn.example.com/repos/MineMineItsMine/trunk
http://svn.example.com/repos/MineMineItsMine/tags/release-2.0
-m "Creating 2.0 release of the 'MineMineItsMine' project."