Connect monitor and keyboard…
sudo nano /etc/network/interfaces
Add the following lines using your own WiFi SSID and PSK PASSWORD
allow-hotplug wlan0
auto wlan0
iface wlan0 inet dhcp
wpa-ssid SSID
wpa-psk PASSWORD
To speed up boot and disable the cabled Ethernet delete these lines from /etc/network/interfaces
auto eth0
iface eth0 inet dhcp
Cheers
Ian