I haven't been to Thailand for about 4 years, but then I took Thai Baht, a debit card for bank machines and two different credit cards. Every where you go US $$ are the preferred currency if you don't have Baht. I'm Canadian and nobody wants that anywhere but here. I use the website www.oanda.com to find the currency converter, they also have a printable wallet size currency sheet so you know how much things cost in comparison to home. When I get to the airport I buy something with the large Baht currency you got from a bank at home and get some small bills and change for things like tips, small purchases etc. Anything larger I put on a credit card, meals, shopping etc. Had no problems with getting more baht if needed from bank machines.
Be prepared for hot and very humid, Thai weather is hot and hotter, it is a beautiful country, the people are very nice.