Hello there,
First off you will need a driver license and an international driving license. You should only rent from a company that has a business insurance in place for the cars; insured for damage, accidents, third party liability.
Often owners of cars rent out cars that have only first class insurance. That is personal and only protects the owner.
Business insurance, only a handful companies have that in place: the international ones and in Phuket I know of a company called RentACar.
Best of luck and safe travels.