Plug the board into your network using an ethernet cable. By default it will get an IP address via DHCP. You can use an app like the Angry IP Scanner (for Windows/Mac/Linux) or iNet Network Scanner (for Mac) to find out what the IP address of your device is on your local network. You may also be able to use the DHCP client list from your network router’s administration page to find this IP address.
For a long term solution, you may want to SSH into the board and set the IP address to a static address for more consistency.