The Thai unit of currency is the Baht. The current exchange rate is approximately 42,00 Baht for 1 US$. You can change money either at the banks in Nathon, or at the money changers in Chaweng, Maenam or Lamai. The basic exchange rates are the same everywhere. It is best to bring traveler's cheques, because they are safer and because the exchange for them is slightly better than the rate for cash. (You have to show you passport and give your address in Thailand if you want to cash a cheque.)
Major credit cards (Visa, American Express, MasterCard) are generally accepted in hotels and the big shops.