If you are younger than 18 years old, please note that in order to obtain a Visa and then the Residency Permit it is necessary that either one of your parents or a legal tutor stays with you in Italy until you turn 18.
About it, please ask for information to the Italian Embassy/Consulate i your country.
If you are less than 18 and alone in Italy you CANNOT: apply for a Residency Permit, register to the Italian National Health System, sign a rent contract and have a bank account.