Welcome to the Cisco Learning Network!
I understand you want to start learning about API's. You might want to start by checking the API introduction video and courses available for you at the following links:
Finally, if you want to review the New Cisco DevNet Associate certification coming out on February 24th, 2020, you can check the following links:
I wish you all the best in your Cisco studies.
Here is a good place to start:
Also, just download and install Postman and Google for API guides for different vendors. Here is the Google Drive API guide:
Try a few examples. Try to define for yourself, what you need the APIs for. What do you want to achieve. Let's say you want to automate your Cisco FTD setup Look for FTD/FMC APIs.