Go to your Users/YourUserName/Zomboid/Workshop

That's where it all begins. Next I'll explain how to set up an empty mod that you can start doing stuff to and with.

Create a new folder, let's name it "My First Mod" (obviously without the " ")
In this folder create another new folder, named "Contents"
In "Contents", create a new folder named "mods"
In "mods", create a new folder named the way your mod's gonna be named. For this example the folder's gonna be "My First Mod"
In "My First Mod", create a new folder named "42", and a new folder named "common" (don't even worry about the common folder, it can stay empty since we're in B42)
In 42, create a new folder named "media"
In "media", you're going to need a new folder for everything you plan on adding/modifying, such as "clothing", "lua", "models_x", "scripts", "textures", etc.

Now you have all your folders ready to go, so let's set up a couple of mod.info files so you can actually enable your mod ingame. The mod.info file has to be an actual .INFO file, so just renaming a .txt file won't be enough. You can copy a mod.info file from Zomboid/mods/examplemod.

The mod.info file can be set up like so, from top to bottom:
name=My First Mod
id=MyFirstMod
description=This is where you can describe what your mod does.
poster=
icon=

You can leave the poster and icon empty, or get yourself some .png files to use there. The poster can be a 256x256 png and the icon can be a 32x32 png. NOTE: Before you can upload your mod to the Steam Workshop, you're going to need a preview.png at Zomboid/Workshop/My First Mod. The preview.png can be a 256x256, so you can use your poster as your preview, and vice versa.

The first mod.info file needs to be at Zomboid/Workshop/My First Mod/Contents/mods/My First Mod.
The second mod.info file needs to be at the 42 folder.
Both should be 100% identical.

When it comes to sub-folders in your Zomboid/Workshop/My First Mod/Contents/mods/My First Mod/42/media folder, you can look at how they're set up in Project Zomboid at Steam/steamapps/common/ProjectZomboid/media. So for example, if you plan on writing your own LUA scripts, you'll need the correct folder(s) for them in your media/lua folder (client, shared, and server). The same thing applies for any 3D models you want to use and their textures, your own custom animations, their AnimSets, etc.