You could certainly start by creating a test that will call an API endpoint. In order to achieve this, an HTTP Request will need to be made to a controller. The controller will then call into a service within the business layer, which in turn will make a call to the repository, and finally a command is sent to the database. That feels like a lot of moving parts. Perhaps there's a better place to start.

In order to break down the problem into smaller, more manageable pieces, perhaps it's best to start testing closer to the persistence layer of the application.