Were do you pick up your car when arriving in terminal S

When leaving the south terminal, there are signs to guide you. Head out the doors where the taxis are, and take the ATL SkyTrain to the Rental Car Center. Two stops, and you're there. Hertz, Avis, Enterprise and Sixt are on the third floor, all others on the second level.

Baggage claim has a sky train that take you directly to the rental car center, all within the airport complex.

There is a rental car center that is off airport. You take the sky train from the baggage claim area to the rental center.