Being a dev and making mods are two related but distinct skills (you can be an accomplished developer and not be a ROM hacker, and vice versa, though there is a lot of overlap.
For developing your own code you have a few approaches.
Low level stuff for the wii probably means https://devkitpro.org/ (with the devkitppc aspect of it -- devkitpro as a whole covers a lot of things).
This will be the fastest and tightest code if you can handle it, however coding in C and C++ is not the easiest thing if you are new to all this.
If you would rather something more sedate then there are higher level languages that are easier to learn and code with, however you get to skip your dreams of making some kind of 3d world RPG that is also a kart racer. Not sure what the complete list is these days but python https://pypi.org/project/Wii.py/ and lua https://github.com/carstene1ns/lua51-wii are available and represent two of the bigger options.