The official currency of the United States is the U.S. dollar ($, USD, US$).
In San Francisco you will find bills of 1, 2, 5, 10, 20, 50 and 100 dollars and coins of 1, 5, 10, 25 and 50 cents and 1 dollar values (100 cents = 1 dollar).

If you are one of those who likes to buy everything with credit cards, you will not have any difficulty finding ATMs and banks in San Francisco.
If you live abroad, the best option to save on fees is to pay with a credit card whenever possible. Luckily, except for street vendors, you get the feeling that you can pay with a card in any store or restaurant, even if the amount is less than 1 dollar.
When paying with credit card the current exchange rate is used and the maximum fee, depending on the issuing company, is usually about 1%.
If you prefer to exchange your money for dollars and pay in cash at the cashier, you can do it either in the banks or in a currency exchange house.
The best option is to check the fees that your bank charges for cash withdrawals in foreign currency.
Here you can see the exchange rate in the main currencies: