The times I have gone, I have parked at a MARTA station and rode the train. Get off at the North Avenue station and walk about 3 blocks. Goes by the Varsity on the way if you are interested in eating there. Only time I have not done that, I parked at GT's McCamish Pavillion. Walk from there is about 6 blocks I think. Cost for parking was fair. That was several years ago so not sure about parking there currently.
If you want to tailgate with Clemson folks, most will be at the Varsity parking lot but you need to arrive early. If you just want to park at a convenient place with a short walk, I suggest the Centergy parking deck on fifth, just across the I85/I75 connector from campus - very easy walk to campus and stadium. You can buy an advance pass for this deck from the GT parking site, or pay $20 day of game.