All main credit cards, like VISA, MASTERCARD and debit cards are generally accepted. AMEX and AMERICAN EXPRESS cards are accepted in all major hotels but may not be in some stores due to the high transaction fees. So that makes it very convenient. Just make sure that the payment process is done in front of you and hide your PIN number. Never ever give your card to a person if you cannot see the full payment process.
If you prefer to pay cash that is fine too but be prepared that the shops will struggle with change, especially in supermarkets. It can take longer to get the correct change than the shopping process. The currency used in Brazil is the Real (plural: reais; pronounced “hey-all” and “hey-eyes”) and comes in the following denominations: