AT&T's international roaming plans are expensive but easiest for the tech-challenged, if you must stick with AT&T. Your phone will work in Italy like it does at home. If you use the $60/month plan, you get 1GB of data to use and local calls to Italian numbers cost 35 cents/min. Or if you use the $10/day plan, you get free local calls and simply use your talk/text/data from your home plan. If your trip is longer than six days, obviously the $60/month plan could be cheaper.
If you want to buy a SIM, you need to get your phone unlocked (AT&T will unlock if your phone is paid off). But you will not have your US phone number while you have an Italian SIM in the phone - you'll have an Italian number until you put the AT&T SIM back.
If you could switch to Sprint or T-Mobile, they have much cheaper international roaming plans: unlimited text and data and 20 cents/min local calls. Sprint has a great deal right now: switch to them and get a free year of unlimited talk/text/data if you have an eligible device. I just switched - it's great ($3.20/month for admin fee and tax).
If you are tech challenged, buying a "burner phone" means learning a whole new device, which you might find a lot more intimidating than the above options.