In my experience Airtran isn't very uptight about carry ons, so take your purse and a small carry on. Make sure the duffel bag meets the specs for carry on. Duffel bags come in many sizes -- most of them would be too large for the overhead bins. I would suggest that you get yourself a small "roll aboard" suitcase -- that way you'll enjoy many more trips in the future with confidence that it will fit into the overhead bins.
If you are worried about getting bin space, sign up for priority boarding with airtran -- it costs a bit extra. But most of the time - especially on the flights to Bloomington, there is no rush and there is enough bin space for everyone.
If you have never flown before you should orient yourself in advance about the Atlanta airport (by looking at a map on the internet).
Have fun on your trip to Bloomington!