MQTT is how the message gets sent. JSON is the format of the message. If you are using Tasmota you don’t have to worry about JSON.
Follow:
- ITEAD Sonoff switches and sockets - cheap ESP8266 Wifi+MQTT hardware
- MQTT Binding (v1.11) Getting Started 101
Look at the log on the Tasmota web page on the switch. It will tell you what it is publishing to which topic.