First of all, if you have a choice, avoid visiting Atlanta during the summer. The temperature is often in the eighties or low nineties, but the humidity is what will really be a problem. Winter temperatures are only in the forties, but the best time to go is in the fall or the spring. If you go in the spring, you might even be able to catch the Atlanta Dogwood Festival.
If you’re flying, you’ll be coming into Hartsfield-Jackson Airport, which consistently ranks among the world’s busiest airports. Delta Airlines is headquartered in Atlanta, and Hartsfield is its primary East Coast hub. There are currently six terminals, with an underground tram that takes you between terminals.
If you don’t want to fly, check out Amtrak trains or Greyhound buses. The local Amtrak station is several miles north of downtown, however, whereas the Greyhound terminal is located at one of the MARTA stations. Check online for schedules and prices; you may even be able to get a ticket on the midnight train to Georgia.
Your final option of course, is driving to Atlanta. Interstates 75, 85, and 20 intersect at the city, with I-20 the lone east/west interstate. I-285 is the loop that circles the city, but the metro Atlanta area now extends beyond the Interstate. If you can avoid rush hour, traffic can be manageable, but otherwise, you will waste a lot of gas and time. If you do make it downtown, there are many parking garages where you can park for the day.
Once you get to the city, you’ll probably want to check out MARTA (Metropolitan Atlanta Rapid Transit Authority). MARTA has two subway lines, one north/south and one east/west, that intersect downtown at Five Points Station. Atlanta mass transit also includes buses, but on a short visit, you will likely be able to get by with just the subway, which extends all the way to the airport.
Fortunately, downtown Atlanta is mostly manageable on foot. You could easily spend two days walking downtown and never even need to jump on the subway. For longer trips, you can get a taxi (there are plenty available), but get a map so that you can walk around town and experience the life of the city that way.