For the average user of the Web, nothing except use a browser which works with XML (see the question about browsers). Remember some XML components are still being implemented, so some features are still either undefined or have yet to be written. Don't expect everything to work yet!
You can use XML browsers to look at some of the stable XML material, such as Jon Bosak's Shakespeare plays and the molecular experiments of the Chemical Markup Language (CML). There are some more example sources listed at http://xml.coverpages.org/xml.html#examples, and you will find XML (particularly in the disguise of XHTML) being introduced in places where it won't break older browsers.
If you want to start preparations for creating your own XML files, see the questions in the Authors' Section and the Developers' Section.