Personally, I use a credit card for whatever I can. However, most US cards aren't chip and PIN, which means you'll have a harder time using them in Europe. What I've found is that major sites will take US cards (I always just explained politely and never had a problem), but smaller areas may not.
Shopping on the street, you'll need euros. I usually just went and ordered them from my bank, but you can also use an ATM in Europe. (Getting them beforehand helped me know what I was spending on vacation.)
Rick Steves' Italy books are very good on customs, tipping, good local food....I'd see if they have them at the library, then order a used copy from Amazon if you like it.
And get stuff together for your passport if you don't have one already!