Here is some information about getting a visa for internships in the USA. You can get information from any of these representatives in Italy.
http://www.ciee.org/representatives/detail.asp?country=IT
We recommend you look at the sites of all these to get information and then contact the one most convenient for any further information.
example_ciee_intern_application is a sample form we filled in for a student in 2007. It may be different now but it gives you an idea of the kind of information and documents you will have to provide.