It depends on what you want to do
- Postman or Insomnia if you want to make exploratory testing on the API
- If you to make some tests to add to your build then maybe somenthing like Rest Assured since you mentioned you have Java experience.
There are a lot other tools out there, but the important part is to start and to understand what and how to test.
You may want to do for you and for fun the 30 Days Challenge for API testing from the Ministry of Testing.
It is fun and you will learn nice things in a non boring way