Getting married, documents for the wedding - Paperwork
Sorrento Wedding Guide

Documents needed from foreign citizens who wish to get married in Italy, vary according to their nationality.
It is advisable to contact a specialized WP agency, which will take in charge all the bureaucratic burden in addition to the coordination of the Ceremony and the Reception in detail.

Generally the following are the required documents:

1 - A valid passport;
2 - A Statutory Declaration (Nulla Osta), issued from the consulate or embassy from the country of origin. it must be authenticated with an Apostille seal at the Prefettura (the lower court), if the country is not part of the European Community;
3 - For foreigners residing in Italy, a "single status affidavit" is required;
4 - Two witnesses (if they're not italian, they must have resident's permit);

Summaries of the legal requirements for citizens from:

U.S.A.

United Kingdom

Australia

Eire (Republic of Ireland)