How can i learn API ?

Answer 1

API is a very generic and broad term, it could mean anything when computer programs communicate with each other.

These days, the RESTful APIs represent the mainstream and utilized by many companies.
https://www.restapitutorial.com/index.html

You can also start learning about it on SL
https://www.sololearn.com/learn/10391/?ref=app

Answer 2

Thanks Tibor Santa