Requirements

For smartthings-mqtt-bridge to work, you must install a message broker.

MQTT broker

Mosquitto is a common broker which works on many platforms.
use google to find more details about setting up your message broker.

MQTT monitor

It is advantageous to use a MQTT monitor initially to help confirm messages are flowing. This is also a useful tool when you want to write and test integration with custom devices.

mqtt-spy is probably one of the most advanced open source utilities for publishing and monitoring activity on MQTT topics. It is suitable for basic day-to-day usage and power users who need an advanced utility for their MQTT-based systems. https://github.com/eclipse/paho.mqtt-spy/wiki/Overview

Smartthings hub

It should be obvious, but just a little reminder, you do need a smartthings hub!