The standard API for handling XML in
Java is called JAXP, and you can find some introductory articles in the
XML FAQ.
The details on how to access the XML data differ a bit depending on where the XML file originates - the client or the server.
Once you have the XML, there are various ways to process them - mainly SAX, DOM and XPath. Which is best depends on how the XML looks like, and what you intend to do with it.