Your best bet is to read other worldgen datapacks for inspiration. Start by doing small modifications to them, and see what they change. Read https://worldgen.dev (which is slowly being moved to the official Minecraft Wiki) as reference documentation. There are various tools, such as Misode's Generators, a bunch of others depending on what you need.
The other place to go is in Discords. We have the #worldgen-dev, where you can ask specific questions, and people will be happy to help. Don't go "please teach me how to do worldgen", because that is very vague and no one has time for that. But asking "how can I make this biome generate more mountainous?" and provide what you have so far is a lot more likely for you to receive help.
Also, keep in mind. Do you want to make a worldgen mod, or a datapack? With a mod, you not only have to learn worldgen, but also the Java programming language, and Fabric/Forge coding. Datapacks are what stuff like Terralith, Tectonic, and WWOO are used by. They don't add new blocks, but they still have a lot of power.
And to respond to your final note, making a paid worldgen course is a terrible idea - it's a lot better for the health of the community to create free and open source tools and resources for everyone to use and learn from. Putting learning resources behind a paywall in the datapack community is considered to be a pretty jerk move. But it takes a lot of work to make these - there's already plenty that exist, you just need to know where to find that. That's slowly changing as we develop them more and more, and as the "advertising" improves.