Simply put, rent a car. Atlanta is not pedestrian friendly. While there is a mass transit system, MARTA, that features 45 stations, there are only two lines – North- South and East-West – so the majority of travel must be facilitated by a convoluted bus system. If so inclined, however, riding MARTA costs $1.75, and it is one of the most (cost) effective ways to travel to/from the airport.
The other option is taxis. The two top companies are Yellow Cab (404 521 0200) and Checker Cab (404 351 1111). The majority of Atlanta is widely spread out, but accessible within a skilled 20-minute drive. Average rates are $2 base plus $1.75 each mile. Within a Downtown/Midtown/ Buckhead zone cab rides are $6 for 1 plus $1 each additional person.