Personally, I would recommend WoWpedia as starting point. You'll find API-documentation and some basic tutorials there.
http://www.wowpedia.org/Portal:Interface_customization
Although, WoWLua is no C#, you'll have to do much more 'dirty work'. There are some templates you can use, but they don't cover huge 'forms' or stuff.
Actually you can view Blizzards Interface code in the FrameXML, for example tekkub has an repo on github.com.