CASH & CREDIT CARDS
To travel to the United States it is practically essential to have at least one credit card. They are accepted almost everywhere and are required as deposit guarantees in hotels and car rental companies. The latter sometimes ask for a second additional card, for example when the car is a luxury category, and the two cards must belong to different circuits.
The main ones are American Express, Mastercard, Diners Club and Visa.
If your credit card has never been used abroad it is always preferable to ask your bank if it is enabled for abroad. Finally, you should check the credit limit.
Last suggestion: bring the useful phone numbers to block the cards in case of theft or loss.
Cash dollars are generally preferable to buy them in a bank in your country because probably the fees will be lower. You can bring currency or monetary instruments worth less than $10,000 or the equivalent of foreign currency per household. Monetary instruments include currency, banknotes, traveller's cheques and bearer instruments such as banker's drafts, stocks and bonds.
Money and Other Monetary Instruments
(U.S. Customs and Border Protection)