How do I activate my existing ATT cellular service to my new IPhone 13

If you did not have your phone preordered to activate to the eSim in your new phone, contact AT&T and have them activate the eSim to your telephone number.

Edit: If your previous iPhone was not a 5G iPhone, and you prefer to use a physical sim card, get a new sim card from AT&T which has been provisioned for 5G service.

Either move the nano sim card form your previous phone to the new one, or setup your service in Settings->Cellular.

More information here: link-> Set up cellular service on iPhone - Apple Support