Do I just put in my SIM card when I buy an unlocked iPhone?
Yes, when you purchase an unlocked iPhone you will be able to use your SIM card in the device. To do this, first make sure that the phone is compatible with your carrier's network and then insert the SIM card into the tray provided on the side of the device. Once you have inserted the SIM card, power on your phone and it should automatically detect and activate your cellular service.