It's always good to have previous knowledge, when learning a new one. But, of course you always have to start somewhere.
The bundled UIBuilder is a great tool. But that alone would probably stop you from going farther than editing (but not for certain). I'm not really sure where's a good place to pickup on XML, the language is kind of funky really, comparatively in my opinion. Sort of different depending on what's using it.
Basically, you can look at it with "tags" that have attributes. And then if you can under stand pages having things inside them (shown in the UIBuilder tree), it's not too hard from there. In editing at least.
A cool tip though, opening your EQ2ui files in FireFox will show if your missing ending pages, just through I'd throw that out there
But yeah, start small, and easy. UIBuilder will definately get your under way. Think there are some tutorial on this forum on getting started.