Oh man, this is opening up a can of worms...
Simply because there are many different visa's and it various from the country you're coming from and it also differs, depends on the province that you're going to.
Pattaya is a province. The rules are different that in Bangkok. Same goes for Chang Mai and all the other different provinces.

Generally speaking if you want to travel here for a vacation U.S. citizens can come here for ONE month with not problem. If you want to come here for an extended vacation. That really isn't a problem either. You can get a multiple entry visa (3 entry) in the U.S. You'll need to pick it up in a major city, (New York, L.A., San Francisco) or do it all through the mail.
Go look at the Royal Thai Embassy (consulate) for information.

Once you get here, when you need things like, you would like to stay here longer, you would like to get a Thai Drivers License, you would like to buy a car or motorcycle, then you might discuss your needs with S.A.M. Visa Express. There are several locations. You can find one in Jomtien on Soy 4, all the way to the end of the soy on the right.

                                           S.A.M. VISA EXPRESS