How do I use the JSON API? Documentation isn't really that helpful.

So I have built an API that I wish to integrate with my chatbot. However the documentation isn't really helpful.
Say, my api is thetestapi.com and I want to send a GET request.
How do I get the input out of the chat? How do I send a GET request, not POST? Also if my API returns something when I send a GET request, how do I get that output??
Even if I wanted to send a POST request, I can't pass the parameters for a POST request in the URL. So how to do POST requests??
Also how exactly should I structure inputs outputs on my GET and POST requests?