Your questions are a little confusing. If you are merely visiting Italy and planning to return to the USA, you can obtain your Carta d'Identita from your Comune in Sicily by presenting your Italian passport and/or your recognition letter from your consulate in the USA and the photos.
If you are planning to establish residency in Italy in a Comune other than the one in Sicily, you would have to apply in that Comune for the ID card by presenting the same proof of citizenship and the photos and either a purchase or rental contract.
To renew an ID card, you merely hand in the one that has become expired. No witnesses necessary.
Also, if you're not an Italian citizen and you are in Italy without a Visa, you cannot obtain an ID card. A Visa can only be obtained from the Italian consulate that has jurisdiction for your address in the USA. Visa's cannot be obtained in Italy.