Yeah, there is no real tutorial of any sorts for map scripting.
A lot of what goes into a map script is pretty much the same as writing a sim-side mod, so you might find some useful information over in the modding info thread
Firstly, you need to familiarize yourself with lua syntax
(and if you're coming from another programming language, you need to keep in mind some differences, such as LUA arrays starting from index 1, not 0!)
Then either download some other scripted maps from the vault (probably the existing TD map), or use the campaign maps as a guide.
As LeVirus said, it's a good idea to get WinRAR or something, and extract the lua.scd file somewhere. This way, when you see a line of code such as "ScenarioUtils.InitializeArmies()" and want to know exactly what it does, you can open up "...\lua\sim\ScenarioUtilies.lua" and search for "InitializeArmies".