As your home country license has expired, you will have obtain your new license the Thai way by going through the whole procedure.
You might find it more convenient to search out a driving school rather than an agent. Many are accredited to the DLT and can take you all the way through from lessons to driving test (theory and practical) and to finally issuing you your license at the DLT. They usually run courses over one week, full time.
It takes away a lot of the hassle involved doing through the DLT, at a cost of course.