Every modding scene starts the same way: a game ships, someone pries it open, and a community forms around what they find. GTA VI has not shipped, so there is no modding API, no file format documentation and nothing to install today — anyone telling you otherwise is selling something.
What you can do now is prepare. Learn the tools that outlive any one game: a 3D package (Blender is free and has carried entire modding scenes on its own), basic texturing, and — if scripts interest you more than assets — a general-purpose language like Lua or C#, both of which have shown up in every major GTA modding era so far.
Study how modding communities formed around GTA V in its first year. The earliest, most useful releases were not flashy — they were people documenting file formats, writing the first extraction tools, and sharing what they learned instead of hoarding it. That groundwork is what a scene is actually built on.
Start a portfolio now. A finished vehicle livery, a clean character texture, a working proof-of-concept script for a different game — anything that shows craft transfers the moment there is something real to point it at.
When the tooling does exist, the modders who move fastest will not be the ones who waited to start learning. They will be the ones who spent this stretch getting good.