We do not recommend renting a bike because of the dangers involved when you are unfamiliar with Thai driving habits, however, if you want to, remember it is just like buying a car. You must be aware of who will treat you fairly on both the rental price or for some minor damage (probably contrived) when you return it. In this area we can save you money and grief.
Educate yourself by visiting this site for some vital information on motor bikes in Thailand. Renting a motor bike.