It may be best to avoid bringing too much money into Thailand, or even carrying it on you while traveling around Thailand. You can get Thai baht from the ATM machines using your major credit card, ATM machines around located just about everywhere, from Shopping malls, to small towns and villages.

The exchange rate is automatic and roughly as good as you'll find anywhere.

MasterCard and Visa are most commonly accepted, but other kinds of major credit cards are commonly accepted as well. Some carry a sizeable surcharge per withdrawal, e.g., $1, so check with your credit card company beforehand if this is bothersome to you.

There are also Currency Exchange outlets, particularly in tourist areas. These are especially useful if you forget your PIN number or if you can't find a working ATM near you which will accept your particular card for some reason.

You should always keep a small amount of cash on you at all times, one reason for this is that the ATM machines often crash late at night, and aren't reset until the morning. Another reason is in case your credit cards are stolen or lost. While Thailand is a relatively safe country, theft can happen anywhere in the world.

Not all ATM machines take all kinds of cards. Many will not take Visa or MasterCard, or will take one but not the other. You have to look for the credit card logos on the ATM signs. There are also the credit card groups such as Plus and Cirrus. If you look around, you will usually find a machine with your insignia on it.

ATM machines offer both Thai and English language.
 
You may be better off paying cash than using a credit card. While it's technically against the law for the Thai vendor to pass onto you the fee that the credit card company charges them (approximately 2.25% to 3.5%, depending upon card type), it's quite common for them to do so, and it's usually not negotiable -- you either pay the surcharge or don't use your credit card. They often want to charge 4 or 5% instead of the 2.25% to 3.5%.
 
You should also be aware that many vendors use a third party to bill your credit card. Thus, your credit card number may not be safe.

Any time your credit card number goes down on paper, you could possibly be at risk.

If you lose your credit card or have other problems, you can reach the credit card companies in Bangkok on the following numbers.

MasterCard (02) 260-8572
Visa (02) 273-1199 or (02) 273-7449
American Express (02) 273-0022 or (02) 273-0044
Diner's Club (02) 238-3660