You may drive in Thailand for 90 days on your home country's drivers license or 12 months on an international drivers license, after that you need a Thai drivers license.
Licenses from most countries can be exchanged for a Thai license and this is not a difficult process if you speak Thai. We can have a native speaker accompany you to ensure the process goes smoothly. This should take no more that a 1/2 day.
Before you can apply for a license you will need to have a medical examination and collect a form from a doctor. This process is basically very simple and in my mind, a waste of time but hey, it must be done. The doctor will charge you about 200 - 300 baht.
Armed with this medical certificate you can go and apply for your license at which time they will have you watch a 1 hour video, check your vision, color recognition and reflexes, take a photo and issue your license. Initially you will be issued a 12 month license and on renewal you will get a 5 year license. The total cost last time I checked was about 500 baht.