When designing RESTful APIs, you should start by listing what the clients of the APIs need to be able to do. For internal APIs, it is straightforward. You can get these requirements by discussing this with the developers of the frontend to find out the interactions that the frontend needs to have with the backend, which usually involves reviewing user stories and UI design work. For public APIs, you need to talk to the developers of the clients of your APIs, ask them their requirements and what problems they need to solve and find out what is common in their needs.