When you make the POST request to create the subscription you get a 202.
Twitch will then send a GET request to your callback.
You need to echo back the hub.callback from the query string.
Once that is done will will get POST requests containing a JSON payload in the body when data occurs/needs to be sent
Parse the payload.
Those of us working in NodeJS are doing this in Express. There are numerous samples on the forum for this (most of them deal with callback paths and verifying the payloads)