The Codice Fiscale is the Italian Tax Code, that serves as a unique identifier. The CF is issued by the AgenziadelleEntrate, and it is necessary for many activities in Italy such as renting an apartment, opening a bank account, getting an Italian phone number, or getting internet set up at home…
You can request a codice fiscale at your Italian Consulate before traveling to Italy (which is strongly recommended).
Otherwise, the CF can be requested upon your arrival in Italy at the Agenzia delle Entrate in Via Ippolito Nievo 36, open from Monday to Friday from 8.45 to 12.45 am (Tuesday, Wednesday and Thursday also from 14.15-15.35). This is the form you must fill in and bring with you to get your CF, and here are the English instructions of how to fill it in.
The office is 500 m from the railway station Trastevere, and it is easily reachable through the 3, 8, 730 bus lines, “Piazza Ippolito Nievo” stop.