What are the cases to be considered while testing a Spring-Boot Web application

I am supposed to develop a comprehensive test suite for a spring-boot JAX-RS web application.

It has following modules

1. config (Which configures the JAX-RS and Redis server)
2. controller
3. environment (accessing details of external servers i.e., which are not included in the application package)
4. exception
5. filter (Request and Response)
6. mapper (DAOs)
7. model
8. service
9. utils

I am new to API developing and I have to a little understanding of REST, Spring-boot, JAX-RS, DAO, JUnit, AssertJ, Unitils etc. which I have acquired in past week. I am new to all of this.

If there could be a comprehensive guide to writing test cases for such kind of applications I would appreciate it. A place to know where to start and how to approach.