Lua is not a simple language in the traditional sense -- it is simple in a minimalist, elegant sort of way. Most people describe Basic as a simple language. Lua is the exact opposite of Basic. IMHO the concepts in Lua take a long time to learn, but those concepts are worth 1000 times more than ASP. (Maybe not in the job market though...
EricTetz cited excellent references. I can vouch for Roberto Ierusalimschy's "Programming in Lua". The book is also available on the Web for free.
I use XEmacs and Lua 5.0 on Windows. Both are Open Source and can be downloaded for free. I also use two computers when developing -- one running WoW and the other for testing+editing Lua code. The WoW directory is shared over the network so it's easy to edit the code and watch the game at the same time. If you can scrape up the $200 needed to have a second computer, you will be amazed at how much easier it is to write WoW code. (The computer I edit Lua code on is "crap" by modern standards. Remember you are only going to edit code and do some light web browsing on this machine. 128MB RAM and a 500MHz CPU is more than adequate.)
My last unsolicited advice is that not everybody is a programmer. If learning Lua is hard and/or boring, don't waste your time on something you don't like. Team up with another person and divide up the job into Frame XML design and programming. Most AddOns really benefit from custom graphics and a better UI. All the Lua talent in the world can't make that happen.