I feel like it’s missing information.
What type of app are you testing? Is it to run in production or for learning purposes?
Whichever testing is being planned you just have a test requirements/test plan before you.
API automation is a tool, like any other tool.
Usually, we test in API - status code returned, payload response - something very specific in it, authentication, negative test - when the API result code is not success, different methods.
Find a JSON path reader as you would need it for verifying your payload specifics.
Learn how to get specific values from JSON path as this is the main method of verifying payload.
Something that I find really useful is being able to interrogate systems using the browser dev tools (network tab) to find API calls and reverse engineer payloads.