Alough it is called XML the style that Eq2 uses is its own version of xml, So the best place to start is in the downloads section right here, Grab a player window something simple to start with and compare that with the default one.. and just mess with it a bit in the UI builder, You will soon pick things up and learn...
Thats how I strated off modding on here, And learn somthing new everyday, Learn by example is how I do it.. Where as I do have a fair bit of programing experience that helps a bit with more complicated issues but not all the time... I think it is safe to say once you learn how to use the UI builder you are 90% there to be able to make your own mods with little or no programing knowlage at all... GL to you