The first Italian fragment found dates back to 960 a.C. and it is the “Placito Capuano”, because it was found in Capua (Campania near Caserta). The text is:

“Sao ko kelle terre, per kelle fini que ki contene, trenta anni le possette parte Sancti Benedicti”

In Italian:

“So che quelle terre, entro quei confini che qui si descrivono, trent’anni le ha tenute in possesso l’amministrazione patrimoniale di San Benedetto”

But that’s just an exception. Latin was still used to write until Dante decided to use his vulgar to write down his works in the 1300. The spoken language was still different at Romans’ times. The real Latin was spoken only in Rome and only by the aristocracy and it was “pure” between the II and the I century b.C..

The Italian peninsula was inhabited by different peoples, when Romans conquered them. These peoples spoke different languages, which had different alphabets and phonetics. These peoples used to speak Latin using their phonetic, so Latin was slowly becoming Italian already in the II century b.C.. It was a slowly change, influenced a lot by barbarian invasions in the V century and by great reigns in the Medieval times.

There wasn’t either a common Italian spoken by people, there were a lot of dialects (and still today) and it’s been decided to use the Italian spoken in Florence because a lot of writers used to write using it.