Creating a mod is as simple as creating a folder inside any valid Mod Directory with a file named Meta.ini inside it.
This file contains information about the mod such as the title, description(s), required hacks and authors.
It should, at minimum, contain a [Miscellaneous] section with a Title property (which the end user will see) and an InternalName property (which is used internally by the Mod Launcher and the mod itself among other things).
Here's an example:
[Miscellaneous]
; The actual title of the mod displayed in the Mod's list.
Title=Example Mod
; The name used internally to refer to the Mod.
InternalName=ExampleMod
To actually do anything with this mod, you will need to require at least one hack.
For example, a mod that just wants to override one file in the game folder would just require CustomFiles and create a folder named "CustomFiles".
RequiredHack=CustomFiles
For more detailed information about configuring the mod itself and the structure of a mod folder, see Configuring Mods.
For more detailed information about which hacks are requirable and which ones are also configurable see the list of Mod Requirable hacks and the individual pages for each hack.