SIM card

The answer is “it depends.” More info here from Rick Steves than can be easily repeated, but it depends on what type of phone and service you have. Is your phone unlocked?

The two largest suppliers are TIM and Vodafone. Both offer tourist oriented SIMs for €30 per month but vary as to to what they offer. On my last three trips, I used the TIM Tourist card. I bought 2 at the TIM counters near the baggage area of FCO and one at a TIM store in Trieste. The other card is the Vodafone Holiday card. A lot of positive reviews on the forum but I have not used it. You’ll have to go to a Vodafone store to get one but they are plentiful and easy to Google. The benefit of TIM is the generous amount (15 gigs) of data. Check what each offers and get the one that best fills your needs and usage.

Yes you can. But before you do, where are you traveling? We just returned from a trip and would have had to buy 5 different SIM cards because of where we were traveling. Italy, Slovenia, Switzerland, Croatia, Bosnia, and Montenegro.

This is not true as I have reminded you several times. One SIM card could have covered Italy, Slovenia, and Croatia (and some would have covered Switzerland also). A second SIM card in Bosnia would have been cheap - mine cost me about $3 USD for about a week's use. (Everything is cheap in Bosnia.) The EU roaming rules went into effect two years ago. Had I taken your trip this year, two SIM cards would have sufficed with my Dutch Vodafone SIM + one for Bosnia.

I have used both TIM and Vodafone in Italy, and like Philip suggests, both are good options in Italy.