The most important document you should have with you at all times when you
travel Italy is your passport or ID. To enter Italy, your passport should have at least 2 empty pages for an entry stamp as well as a 6 months validity remaining beyond the date of departure from any country which is a member of the Schengen area, Italy included.
The good news is that all US and Australian citizens may stay on the territory of Italy for up to 90 days as tourists or for business purposes without a visa. If you live in the USA or Australia, having citizenship of any other country, you need to check whether you need a visa or not.