XML is the work of the devil in regards to addon coding, in my opinion (translate: IMO for you 1337 nerds out there).
Just with any language, start with the basics. WoWwiki has a decent startup guide
here. Follow that up with the "HOWTO" sections for some more specific examples and tips/tricks.
After that, start downloading some "simpler" addons out there and open them in notepad (or whatever editor of your choosing). Examine the code. Try to follow the function flows and see how they work. This, besides for the core basics in the "Getting started", is perhaps the best option for learning as it can show you a variety of ways to do similar things but in different structures. It's my firm opinion this is actually the
best way to learn how to write addons for WoW (though not the best way to learn LUA in general).
There are many others, myself included, that learned from the ground up just as you want to do. It does take some patience and dedication, but once everything "clicks", you'll find it's a lot easier than it looks. As with any coding language, the worst points are where coding can be tedious and repetitive. But in the end, you get a great feeling when it all works the way you wanted to.