See the transportation office near the baggage claim area. There you'll find bus schedules, cab numbers, and car rentals. Or call the service at 404 530 6674.
You can walk to the subway - a train leaves from the MARTA station for downtown every eight minutes, costs $2.5 USD, and takes just 15 minutes.
Buses depart from the lower level of the North Terminal and run from 7 am to 11 pm. You can get there not only to Atlanta, but also to other surrounding cities. The cost of the trip will be about 10 USD, and the travel time depends on the destination.
A transfer to Atlanta by cab will cost 15-20 USD. To order a car, call (404) 762 6087.