If you want to use the Sonoff and control it from Indigo, you need to flash it with the Tasmota firmware so that it uses MQTT. By default, you have to use their proprietary apps.
And that requires opening it up and connecting some wires so the you can connect it via a serial-USB adapter. Which isn't trivial, if you've never done any soldering or other hardware hacking. If all of that is outside your comfort zone, maybe pass this one by.
But if you want to give it a shot, you can use the rPi, which means learning some things about them. Or return the rPi, and get one of these:
https://www.amazon.com/gp/product/B075N82CDLThen you can use the Mac (or Windows) based tools to flash the firmware. You'll still need to solder some wires or headers, because the Sonoff doesn't have anything on the serial connectors you'll want to use.