Mini-UART character output and echo back example for RPi Zero W.
With RPi Zero W we have to use mini-UART because UART0, which is available on other RPis, is used for the on-board bluetooth.
Connect GPIO14(Tx), GPIO15(Rx), GND to your UART adapter Rx, Tx, GND. GPIO14 is Pin 8 and GPIO15 is Pin10. While there are several GND pins, Pin6 is recommended since it is easy to find.