Mainly you need a text editor (text editor with color coding is better though) to edit SkinDefinition.xml and Photoshop (or Gimp or something similar) to edit .tga pictures
Gimp
Notepad++
There were some nice tutorials on this site too. Try:
http://www.lotrointerface.com/index.php?p=guides
Tutorials & Other Helpful Information
Here's a good tutorial how to handle TGA files:
http://www.lotrointerface.com/forums...read.php?t=558
You will also need the files which you can then edit:
Moria Book 8 UI Skin Art
-d