@victor hugo and @unwind are correct, and victor's solution is by far the simplest. However BEWARE of externals in your SVN project. If you reference external libraries, the external's revision reference (whether a tag, or HEAD, or number) will remain unchanged when you tag directories that have external references.
It is possible to create a script to handle this aspect of tagging, for a discussion on that topic, see this SO article: Tagging an SVN checkout with externals