If you need to check in to your 2nd leg and to pick and drop your checked luggage, you need the TDAC. If you're coming to Bkk checked through, you won't leave the departure area and therefore you don't need the TDAC.
Go to the Advance driving school, next to BPH. They will guide you through the process. It's all inclusive about 2000,- thb, incl practice and theory test.