Lua

LuaGuide
The scripting guide for creating styles, missions, extending rules using text editor.

What is Lua?
You can find an amazing tutorial here: http://tylerneylon.com/a/learn-lua/. You'll be ready to code in Lua in less than 15 minutes!

References
Now that you are familiar with the coding tools, you can read the documentation for the various parts of the engine.

Lua API documentation, GearTypes, VisualGearTypes, Sounds, AmmoTypes
You could also try checking out the Hedge Editor.