Atlanta is a city in Georgia (United States). In terms of area, it's huge and extremely spread out, making it somewhat difficult to hitch out of. All but one Interstate in Georgia runs directly through the center of Atlanta, so you have a fast route to any major, surrounding destinations.
There's really no distinct "center" of the city, but hitching out from anywhere inside the I-285 perimeter is unlikely. It's highly suggested that you take transportation to the station that is furthest in the direction that you want to go, and then you may want to take commuter buses even further, as the transfer is free.
North towards Dahlonega (GA-400)
Take MARTA north-south rail line to North Springs station, N11, and from there take a bus (free transfer from the train) further north to Alpharetta or Roswell where there is dense traffic but you are well out of the city and into the suburbs and it's a lot safer. Alternatively, you can leave the station at North Springs and try to get rides with people as they leave the parking garage. Most, if not all, people will be headed north.
North-east towards Greenville,SC (I-85)
Take MARTA north-east rail to Doraville Station.
Legendary Rob suggests the Pleasantdale Rd on-ramp (Transfer to bus 124 from Doraville Station). I found this spot unsuitable as it had no wide shoulder on the onramp. It was only possible to solicit ride from cars arriving from one direction as they slowed for the right-hand turn. As this exit is so close to the city centre, most drivers are local workers and shoppers.
There are excellent wide shoulders at Pleasant Hill Rd (Exit 104 - Bus 10 from Doraville Station and a bit of a walk, though a good spot) and Discover Mills (Exit 107). Seek a ride to at least Commerce, GA. Also, ask people to take you to the Mall of Georgia, a few exits away. It draws many long distance drivers for gas, shopping, and food. Asking in the stations here can yield some nice, long distance rides.
South and others (I-75)
If you're stuck in the city, there's a nice spot (complete with red light) at the corner of 10th St. and Techwood Drive where you can find cars entering both I-85 and I-75.
Love's truck stop - Truck Stop mecca on I-75 south of Atlanta, exit 201.
Atlanta is served by the Metropolitan Atlanta Rapid Transit Authority or MARTA. There are three rail lines that are important, the north-south line, east-west line and the northeast or Doraville line, as well as numerous bus routes. A single fare is $2.50. You'll also have to buy a $1 Breeze Card the first time. Buses are regularly late.
You'll most likely need to use public transportation to get out of the city, but be cautious. At night, you may want to avoid leaving the train at these stations: Vine City, Bankhead, East Point and College Park.
Sleeping in parks and anywhere else in the city without accommodation is highly unadvised. If you ever find yourself needing a place to stay, get in touch with Zac. If that fails, You will be best off finding a quiet place to crash in the Buckhead Community, which is the northernmost area of the city limits. Please note that if you search for Buckhead, Atlanta on Google maps, by error it gives you Bankhead – somewhere you really, really don't want to sleep.