Important: tell your bank a few days before leaving for Thailand to prevent it from blocking your card. By seeing withdrawals from Thailand they may think your credit card was stolen and block it.
You can usually pay by credit card in the big hotels and big restaurants, department stores and supermarkets. But always have cash on you for the rest. Thai people are use to get paid and pay in cash.
Each desk has an exchange rate for the purchase and a rate for the sale of a currency and they vary depending on the change counter. In Thailand, Superrich is renowned for having the best rates, but they don't have branches everywhere. There are present at the lowest level at Suvarnabhumi Airport, near the Airport Rail Link. I can't really tell you if it is better to withdraw bahts several times during your trip or to come to Thailand with a large sum of money and change it in baht when you arrive in the country. Changing will cost less but you will have a large amount of money on you all the time which is not always easy to manage especially against the risk of theft which is not high in Thailand but still. Maybe the best solution is to mix between money you change when you arrive and some withdraws during the trip.