I have invitation letter from Italy so how I can...

you can obtain visa  to Italy  through the official website of the ministry of foreign affairs in Italy and you need to provide the following documents:

-ID photo

-valid passport for not less than 3 months

-round tickets

-hotel reservation, in case you have a relative in Italy, then he or she must guarantee the place of residence during the whole period of trip .

-prove that you have enough money for the expense during the trip  such as, (bank statement or salary receipt or the ownership of a house)

- the tourist must be able to spend between 30 to 50 Euros in a day

- medical insurance covers the whole period of the trip through a bank guarantee or one of the insurance agencies