Under the 60-day visa waiver programme for certain nationalities travelling for tourism or business, you do not need to obtain a visa before travelling or on arrival. In other cases, airlines will ask you for a valid visa to board.
Only these nationalities can travel to Thailand and obtain a 15-day visa there : Bulgaria, Bhutan, China, Cyprus, Ethiopia, Fiji, Georgia, India, Kazakhstan, Malta, Mexico, Nauru, Papua New, Guinea, Romania, Saudi Arabia, Taiwan, Uzbekistan, Vanuatu