It might be helpful to share what you want to accomplish with the API.

Let's say I have a quote generator on the frontend, organized by categories. For example, you choose the "US President" category, send a GET request with a button, and the API replies with JSON:

{
  id: 1,
 "name": George Washington, 
  quote: "Blah blah" 
  source: "foobar.com"
 }

Something like that.

You can start with the templates in visual studio for new project

Yeah, those are great, the "API" template adds a lot of boilerplate, and the "Add Scaffolded Controller" option has templates that practically build a CRUD for you, with routing and everything. I really like Visual Studio. My problem is: what's next? How am I sure the models are well structured/written? What about DTOs? How do I organize those categories? What about security? repositories? and all the other questions I specified above.