The tools you will need are fairly simple. First, you will need a language reference. I personally use http://www.lua.org/manual/5.1/
which is fairly easy to understand and navigate.
Second, you will want the Turbine API documentation. As of the time this guide was written, the lastest API docs were published on the LoTROInterface.com website at http://www.lotrointerface.com/downlo...mentation.html
You will need an editor, a simple text editor like Notepad will suffice but some users prefer syntax highlighting editors or project managers to organize their files (I generally just use Notepad).
If you plan on using any custom graphics, you will want an image editor that can generate .jpg and/or .tga files as these are the only file formats that LoTRO Lua will display.
The last thing you might want are some sample plugins to dissect and play around with. Turbine published a package of sample files which can be downloaded in a 7zip archive from http://content.turbine.com/sites/lot..._LuaPlugins.7z
You may also want to check out LoTROInterface.com or other plugin sources. One of the best ways to learn is to dig in, twist, pull, yank and turn and see what happens :)
One rule to bear in mind, most things dealing with Lua are case sensitive so if you keep getting a nil value or an error that a function doesn't exist or any other mysterious error, always double check that you have the correct case.