How to read root element in XML DOM in Salesforce?

Hi,
Apex provides classes that enable you to work with XML content using the DOM (Document Object Model).
DOM classes help you parse or generate XML content. You can use these classes to work with any XML content. One common application is to use the classes to generate the body of a request created by HttpRequest or to parse a response accessed by HttpResponse. The DOM represents an XML document as a hierarchy of nodes. Some nodes may be branch nodes and have child nodes, while others are leaf nodes with no children. You can parse nested XML content that’s up to 50 nodes deep.