Italy is part of the Schengen Agreement, meaning that entering Italy from most other parts of the EU is pretty easy. There are no border checkpoints or customs. Document and customs checks remain standard if arriving from (or departing to) a non-Schengen country.
If you are an EU or Swiss citizen, you can travel to Italy with your national identity card alone. All other nationalities need a valid passport. Visas are not generally required if you are staying less than 90 days (or at all for EU citizens), for citizens of countries including, Australia, Canada, Israel, Japan, New Zealand, Switzerland and the USA.
If you are entering Italy for more than 90 days or for any reason other than tourism (such as study or work) you may need a specific visa. See vistoperitalia.esteri.it or contact an Italian consulate for details.
Ensure your passport is valid for at least six months beyond your departure date from Italy.