To make your own mod/plugin you will need to learn to script in the LUA language. Its fairly simple to learn.
I know of 3 great sites for learning LUA which are:
http://lua.gts-stolberg.de/en/Variablen.php
http://lua-users.org/wiki/LearningLua
Lotrointerface.com also has a wiki designed around the LOTRO API and some brief description on how to make plugins for the game at
http://www.lotrointerface.com/wiki/LoTROInterface_Wiki
Good Luck, have fun, and keep us posted on your achievements
