Travellers entering Thailand need to obtain a visa according to the purpose of their visit from Royal Thai Embassy or Royal Thai Consulate-General. However, nationals of some countries may enter Thailand without visa or are eligible to apply for visa on arrival for a certain period of time, and for certain purposes.
Check the list here for the list of countries entitled for visa exemption and visa on arrival.
For more information on visa requirements, kindly check the Thailand Immigration Department website: www.mfa.go.th