The Codice Fiscale is a personal tax account number, needed to obtain certain documents or make extraordinary expenses. The number remains the same even if you return to Italy after many years. Steps to obtain one:
1. Bring your Passport and a photocopy of your passport data page and visa.
2. NO NEED: photocopy receipts from Posta (the ones for Soggiorno application).
3. Go to the Agenzia dell’Entrate, Via Aurelia, 866 (after Aurelia Hospital, past Panorama and other stores). If taking 246 or 247 bus, get off at Panorama, and walk a bit more. Fall in line 15 minutes before they open in the morning so there are less people and your application can be processed right away. See schedule and other information at: http://lazio.agenziaentrate.it/?id=uff&iduff=TK2&codereg=913&iduffreg=TJP
(Just check if the website is updated, as the office was formerly at Largo Lorenzo Mossa, 8, 00165 Roma.)
4. Ask the information desk for the form you are to fill up, then get a number and wait for your turn. At the end of the application process, you will be given a sheet with your Codice Fiscale number, which you can immediately use for legal purposes. The actual card will reach you by mail some days later.