Help with functor

how can I turn prolog rules into functor?

Please provide more details in your question. According to the glossary here a functor is a “Combination of name and arity of a compound term. The term foo(a, b, c) is said to be a term belonging to the functor foo/3 . foo/0 is used to refer to the atom foo.”

The rule processo(minorenni(T,U)), is a functor?

 processo(minorenni(T,U)):- testimonePartecipaudienza(T,U),
    			   getdataUdienza(DU,U), 
    			   getdataNascita(DT,T),
    			   S is (DU-DT), S < 18, 
    		           testimone(T).

In this case you would say that the functor is processo/1; i.e. you have a compound term with name processo and arity 1.
/JC