Turner Field is located at 755 Hank Aaron Drive SW, closest to the Georgia State MARTA station on the blue/green rail line. Walking time is about 25 minutes and a very easy route. Merely take the Piedmont Avenue exit out of the station and turn left and walk south until you reach the stadium.
However, most people going to a Braves Game take the Braves Shuttle (click for more information), which picks up passengers next to Underground Atlanta.
Bus 55 and bus 32 also depart from the Five Points Station and go to Turner Field. Plan ahead and look at the bus schedules online to know departure times, or use Google Maps or one of the MARTA apps on your smartphone for trip information and schedules.
For more information, see:
How to Ride the Braves Shuttle
Navigating the Five Points Station
Navigating the Georgia State Station
MARTA Train Schedules and Frequency
5 Tips for Taking MARTA to Braves Games
5 Common Mistakes Made By Braves Game MARTA Riders