Plug and boot your server up

Plug your server in wired Ethernet, or configure the wifi connection as explained here. You can also mount the second partition of the SD card and edit the wpa-supplicant.conf file prior to boot the card for the first time - on Windows you can use Paragon ExtFS, don't forget to unmount everytime for changes to take effect.
Optional : You can connect a screen if you want to see how boot is going, and a keyboard if you want to have a command-line access to your server.
Power up the server, wait for the first reboot to happen, and then wait until you see a big squared Y :

Write down the IP address field visible on the screen: It is your server's local IP address.

Don't worry if you don't have a screen ! You can still find you IP address in the next step.