Well, what do you intend to use XML for?
XML is primarily a format for defining languages -- XHTML is one XML language which is also HTML 4.01. RDF is an XML language for describing data (metadata, basically). You can create your own generic XML language to contain data specific to your application, whatever that is.
The most important question is what do you plan to do?. Answer that question, and we can tell you whether XML is right for you or not.