In Civ, you have 3 choices of where to mod: XML, Python, and DLL. Some are harder than others, and some things can only be done in one of them. Depending on what you're trying to make, you might need any combination of the three. However, there are some things you need to know about how mods work in Civ 4 first.
What is a mod?Edit
A mod, put simply, is something that changes the base game to improve or alter it to suit a particular purpose. For Civ, we have a range of mods going from "total conversions" like Fall From Heaven 2 which change almost everything about the game, and also mods like Varietas Delectat, which just adds some nicer graphics.