To make a terminal ready to accept payments, it needs to be assigned to your store, and then boarded. Often the terminal is already assigned to your store and all you need to do is confirm. But it's also possible that you need to select your store from a list on the terminal display.
Board your terminal
Switch on the terminal. During startup, the display turns on and off a few times.
If the display shows Configure network, connect the terminal to the internet.
On the terminal, select Board the terminal.
Follow the instructions for your situation:
The terminal reboots one or more times. Your terminal is now boarded and ready to process payments.