Most people going to a Braves Game take the Braves Shuttle. However, you can walk or take other buses. The closest MARTA station to Turner Field is actually the Georgia State station on the blue/green rail line. Walking time is about 25 minutes. From the station, turn left on Piedmont Avenue and walk south toward the Capitol.
Bus 55 and bus 32 depart from the Five Points Station and also go to Turner Field. Plan ahead and look at the bus schedules online to know departure times, or use Google Maps on your smartphone for trip information and next arriving bus when going to or leaving the game.
For more information, see:
How to Ride the Braves Shuttle
Navigating the Five Points Station
MARTA Train Schedules and Frequency
Using Google Maps for MARTA Directions
5 Common Mistakes Made By Braves Game MARTA Riders