Cards work almost everywhere
Most shops, taxis, Uber, restaurants and transit accept Visa and Mastercard. Many accept tap-to-pay with your phone. Before you fly:
Tell your home bank you are traveling, or check the app for a travel setting.
Check the foreign transaction fee (an extra percent added to each purchase abroad).
Bring two cards from different banks, in case one is blocked.
When a card machine asks "pay in US dollars or in your home currency?", choose US dollars. The other choice usually has a worse rate.
An ATM is a cash machine. Use ones inside banks or airports, not small machines in shops. The ATM owner usually charges a fee, and your home bank may add another. Take out a larger amount once, not small amounts many times.
Bring a small amount of US dollars for the first day, in small bills ($1, $5, $10, $20). Some places will not take $100 bills. Airport exchange counters usually give poor rates; exchange only a little there.
If you or your family together bring more than $10,000 in cash or similar (traveler's checks, money orders, some checks), you must declare it to CBP when you arrive (FinCEN Form 105). Bringing money is legal. Not declaring it can mean CBP takes it.
Open a US bank account in your first weeks. You often need your passport, your visa or I-20, and an address. See Opening a US bank account without an SSN. Moving larger amounts from home later is cheaper by bank transfer or a money transfer app than by cash.
Relying on one card. Banks sometimes block a card at the first foreign use.
Carrying all cash in one place. Split it between your bag and your wallet.
Choosing "home currency" at the card machine.