To make a terminal ready to accept payments, it needs to be assigned to your store, and then boarded. Often the terminal is pre-assigned to your store and all you need to is board it. But it's also possible that you need to select your store from a list on the terminal display during the boarding.
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.