ts the SIM card slot - if you bought the phone locked to a carrier like ATT there should already be a sim card in the phone - all you need to di is activate it - if you bought it from Verizon or Sprint they dont use SIM cards
this is assuming a US iPhone - for other contries if you bought the phone locked to a carrier a sim card for that carrier should also already be in the phone - if you bought an unlocked iPhone 9in countries where they are sold unlcoked) you just need to contact a supported carrier to get a SIM card
If there is a new never used SIM Card in your iPhone 4S, you need to call ATT and tell them the ICCID# of the SIM Card, it is on the iPhone Box Label. Then ATT can turn ON this SIM Card. If you iPhone is functioning, you can also look up the ICCID# by, tapping Settings App > General > About > ICCID the number is right there and you communicate this number to ATT. If this is not your SIM Card or your iPhone does not have a SIM Card you will need to go to ATT Store to get a SIM Card and they will help you getting it working right there.