You will need to visit a local AT&T Company Store to receive a free sim card and to add it on your plan.
You also want to have the store check to make sure that the 4S is unlocked. You can do this yourself if you have a micro SIM card by placing it in the iPhone. The SIM tray is on the right side of the device and you will need a small paperclip or earring to pop open the tray.
Hope this helps!
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