Italy uses the Euro like the rest of Europe so be prepared to exchange some money at the airport to get you out and on your way, but only get enough for a day or so because the exchange rate is terrible. Once you are out of the airport look for an ATM they are plentiful and get your money there. We also have an ATM in our little village when you get here. Don't expect to use USD anywhere as it will be refused.
Also Italy has a VAT tax which is sales tax and it's on everything including food and services and it's 21% so don't be surprised. Good news since you will be tourist's if you make purchase's of goods over $100 per item you can get from most stores a VAT refund envelope. It MUST contain their information and your original receipt and then when you get to the Rome airport to leave you go to the VAT refund center and get a stamp on your paperwork and depending on the store you purchased from you may get immediate refund on the spot or get it returned to you on your credit card by mail. I have done both in the past and it takes awhile but it's nice to get the 21% back.
Of course you need a US passport to enter the country but the GOOD news is you get an automatic 90 visa, usually as soon as they see the blue cover they don't even bother to open it. At first I use to get disapointed that I didn't even get a stamp.