I started doin some messin around not too long ago myself. Modding the default UI is the easiest way to get started, IMHO.
I first started playing with the layouts, sizes, types of boarders...etc. Then tried adding different elements from other windows. (FYI- some ya just can't move...well..and have it work in game anyway..heh) Now I'm starting to play with some re-texturing which involves adding scripts to the base XML.
There's some awesome tutorials in this forum...I definately check them out. I play with SIDL and pretty much nothing else. It's relatively easy to use. All ya really gotta do is learn what the different things do. There's a wealth of info in the tutorial section. Oh...and make the "EQTypes for everything" your best friend!
I'll try to find the link to the SIDL app for ya...oh..and you gotta have the Microsoft .NET Framework in order to ever boot up SIDL...so get that first.