The codice fiscale is a financial identification number, used in Italy for everything from buying a mobile phone to opening a bank account. If you are planning to live in Rome for any length of time, having a codice fiscale will be vital. Without one, you won't even be able to register a prepaid mobile phone.
How to get a Codice Fiscale
The codice fiscale is issued on a small green card.
The codice fiscale is calculated based on your name, along with your date and place of birth. In theory, it's easy to get a sort of codice fiscale: there are even websites where you can punch in your information and get a codice straight back. However, these aren't 100% legitimate. There's always a chance that two people might have the same information, and so the same automatically generated codice.
While an automatic one might be enought to get by, only an officially issued codice fiscale can really be considered the genuine article, and these are issued by the regional offices of the Agenzia Entrate, who have an English-language section on their website at agenziaentrate.gov.it.