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. My husband has done it twice and both times the doctor only wanted to examine his hands and legs. No questions about medications you take or any other impairments you may have. The doctor will charge you about 200 baht ($6.00)
Armed with this medical certificate you can go and apply for your license at which time they will 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 ($15.00).