Sounds like you will be coming into Atlanta on I75 southbound and need to go I20 eastbound to Augusta. The only exit you will need to take is the I20 exit just as you get south of Grady hospital. There are 2 (maybe 3) right hand lanes that are part of the exit onto I20. It is not a left lane exit. The I20 lanes run a fairly long way so it is more than just an exit ramp. That should make getting into the lanes easier. Just watch for some right side exit only lanes as you go through downtown. Maybe try to stay in second or third from right lane as you get into downtown. They have painted big interstate looking onto the roadbed of lanes so you can see which interstate you will end up on as they split after going through downtown.
BELOW IS FOR TRAVELING AUGUSTA TO ADAIRSVILLE...YOUR TRIP HOME???
Assuming you will be traveling back through Atlanta headed to Adairsville, when you are on downtown connector north of downtown traveling northbound, the I75 & I85 lanes cross when they separate. So, even thought I75 runs west (to the left) of I85, you will need to be in the right hand lanes.