Wanting to build addons

[Drumstix42]

Well, the quick and dirty reply I guess would be to pick up UIBuilder from the EQII folder and try messing around with some of the windows. If you're not code savvy, this is probably the easiest way to go about it.
I find it best to just pick something out, modify something simple, and see how it works in game. Maybe afterwards, take a custom window, and compare the differences to default. Pretty much one step at a time.

Take a look around at what info other interface authors have posted, any available How-To's, and I guess looking up how XML is usually formatted can help a bit. Asking questions are good, and there's also plenty answered around the forums.