Where to find the records on line
Start at http://www.antenati.san.beniculturali.it/?q=gallery for a list of provincial archives. If your ancestral province is listed, click on it. If not, try familysearch.org. Otherwise, sorry, we cannot help you. That is the scope of this guide.
If your ancestral province is listed, click on the link, and then find your ancestral town. Remember that the town or province may have changed its name. For example, if your ancestors lived in Principato Citeriore, search under Salerno province. On each page of towns, use your browsers search function to search for the town name, which might be in parentheses after the word oggi which means today. Note that while standard genealogical practice is to use the place name in vogue at the time of the event, FamilySearch might not accept it, and may want the current town name.