If you have a foreign license already, it is quite easy to obtain a Thai license. This is what you will need:
Work Permit (with home address) OR Certificate of residence from Thai Immigration (you can see the requirements for that [here]. The government cost is 1000 THB. We can arrange this paper for you. See the services page for details
5 diseases health certificate which you can get for about 50 THB at almost any clinic (must use within 30 days of issue)
A printed description of your license classifications from your Country/State’s DMV/DLT department website. (ie, M1, C, etc), as well as a copy of the front and back of your license
Signed copy of your passport (picture page, current visa page, TM-30, and departure card)
Fee: The cost for the car drivers license card is about 205 THB per card (car and motorcycle license cards are issued separately)
If you are going to do a motorcycle license too, you will need to make another copy of all your documents as well.
Initially you will get a 1 year “temporary”, then you will have to come back to renew with the same paperwork again and the fee will be 605 THB for the 5 years card.
If you do not have a foreign license or it is expired, in addition to the paperwork above, you need to be there at about 8:30 to sit through a 4 hour training course (in Thai so take a translator if you want), then take the written (English available) and driving tests (you need to provide your own vehicle). Be prepared for a full day event.