The entire United States uses the same currency: USD, which uses the symbol $ in front of any amount. Bill denominations are $100, $50, $20, $10, $5, and $1. The less-common $2 bill may be used from time to time. Coins are available in 25¢, 10¢, 5¢, and 1¢; occasionally a $1 coin may fall into your hands.
Exchange facilities are available at airports, malls, and various other locations searchable online. It is advisable to check the daily exchange rate and that which businesses offer prior to a transaction. Because ATMs are widely available, it is typically better to withdraw money from ATMs - check with your local bank if there are US partners, which allow for waived withdrawal fees. Similarly, credit cards are widely accepted and several companies offer cards that waive international transaction fees. Travellers cheques are increasingly less popular, but some large businesses accept them and banks will exchange them for cash.