Do you want to just write an XML file or read XML using a particular language?
XSL is used to translate XML into XHTML.
These tutorials form w3schools.com might help.
XSL XML
I have a tutorial on using XML and the ASP.NET repeater control here
A while ago I wrote a blog in XSLT and XML http://www.whatisreal.com/index.aspx You can view my source using the links to the right. That should get you started.
Best way to learn I found is using the online resource and a tool that can edit and understand XML (something that will guide you in the development). The visual studio express tools are free and can do it. This is a really good source also stylusstudio.com but not free.
This may be another good source of information msdn.microsoft.com/en-us/xml/bb291061.aspx