Don't know of any free program, but XML Spy is where I learned all about XML here.
I just grabbed a small book/print out from online, read about it. Then got XML spy ( from work.
). And played with it.
Loaded a XML document, created a DTD file ( this is something that VI really needs ) and vis versa.
Play with it, and you'll learn. Its really a nice app and XML Isn't that hard. It's all structure, nothing more.