There are much better mesh networks to setup but the simplest to get started with is two meshtastic devices and start having a play. 

The HeltecV3 boards (ESP32 with LoRa) is easiest to get started. 
The board is like ~$25 from many vendors. 
Can connect to MQTT server and get your messages broadcasted on public internet if you want
- I still need to do this. Trying to hone this in for the best privacy-focused setup I can have (MQTT isn't very private since the trade off is pushing data out to clearnet).