Unless the new phone is locked to another cellphone company you should just be able to pop in the SIM -- you may have to contact AT&T and get them to update the phone afterwards -- which they can usually do remotely.
If AT&T have main street stores (I'm in the UK and the big cellphone companies have stores here) I'd just get their staff to check the phone do it all for you.
If the phone is secondhand and is locked to a previous provider, there are corner stores and electronics shops who can unlock it -- for a few dollars.
nsert the SIM card into the cell phone. The SIM card slot is often located near the cell phone battery. It is usually only possible to properly insert the card one way. It should not be necessary to use force.