If I am refused, what can I do?

Short answer: Read the letter first. A refusal must be reasoned and motivated, and communicated in a language you understand, or failing that in English, French, Spanish or Arabic (Ministry of Foreign Affairs). The stated reason decides everything you do next.

The appeal route, officially

An appeal against a visa refusal goes to the Regional Administrative Court of Lazio, the T.A.R. Lazio, within 60 days of notification of the decision, and must be served on the Avvocatura dello Stato (Ministry of Foreign Affairs). Family reunification cases go to the ordinary court instead, with no time limit.

What that means in practice

An appeal is a court case in Italy. It needs an Italian lawyer, it costs money, and it takes time. For most refused students it is not the right tool and a corrected application for the next intake is. But the 60 day limit is real, so if you are considering an appeal, get advice quickly rather than after a month of thinking.

What most people should do instead

Take the stated reason seriously and fix that specific thing. A refusal for insufficient funds and a refusal for an unvalidated pre enrolment are completely different problems with completely different answers, and treating them the same way wastes a year.

What never to do

Reapply with documents edited to answer the refusal (V18).

What I do not know

Your chances on appeal, how long a T.A.R. case takes, or whether a refusal affects your next application. Those are questions for an Italian immigration lawyer, and this is one of the places where paying for real legal advice is worth the money.