API testing is a type of software testing that involves testing APIs directly and also as a part of integration testing to check whether the API meets expectations in terms of functionality, reliability, performance, and security of an application. In API Testing our main focus will be on a Business logic layer of the software architecture. API testing can be performed on any software system which contains multiple APIs. API testing won’t concentrate on the look and feel of the application. API testing is entirely different from GUI Testing.

I suggest you learn the API basics stronger. About API, API Orchestration, REST, and SOAP protocols, even if possible try to build a sample API. Once you are good about the concepts, you can perform API Testing any tool provided.