This well-written guide starts with the basics and builds from there. I've seen entirely too many AppleScript guides that either go from the Tell statement right to masters-level stuff or that end entirely after creating the obligatory "Hello, world!" script.
Joshua, writing for AppStorm, covers variables, accessing dictionaries and walks readers through the creation of a handy Mail script. It's not comprehensive, of course, but just right as an introduciton. Check it out and get scripting.