We fly with show dogs in cabin a lot. Have never used AirTran but have heard they are fairly liberal with numbers allowed in cabin and a low fee-like $75 each way. Make your reservation for the dog in advance. Have a health certificate that’s current (easy to get from your vet). Most airlines don’t ask, but better to have one than not.
Potty areas are common at most airports now.
Sherpa makes airline approved carriers. The medium size is pretty versatile and should fit under the seat. They’re available on Amazon, or at Petco/Petsmart. I turn it lengthwise with the opening toward me. Request a window seat or an aisle. I do unzip the top a bit to allow extra air, but never take the dog out. No food in flight, maybe a piece of ice or small sips of water if it’s warm.
If it is a breed that tends to get warm easily (I have French Bulldogs) take a cool coat or a small spray bottle and a portable fan. Once you’re airborne it’s OK but sitting on the ground can get warm
Take your dog for some car trips in the Sherpa to fun places so they become accustomed to staying inside of it when it’s closed up.
Have a great trip!