Italian girls are well-known for being beautiful and glamorous; from the days of the Renaissance muse Simonetta Vespucci through to modern Italian actresses like Elisabetta Canalis, Italian women have been inspiring all kinds admiration for centuries.
To be able to share your admiration for an Italian girl with her, you'll need to know how to tell her she looks beautiful. In Italian, you can use the world bella to denote beauty, with bellisima used as a positive comparative to mean 'great beauty'. Coupled with the Italian word for girl, 'ragazza, you can easily convey your admiration by saying bellisima ragazza with the addition of the adjective Italiana at the end of the phrase if you wanted to specify her nationality.
Although bellisima ragazza Italiana is the correct translation, actually approaching a female with that line would be as awkward as approaching a woman in the US and calling her a 'beautiful American woman.'
In Italian, you can simply use the word 'sei bella' or 'you are beautiful' to praise a girl.
If you're feeling a bit cheekier you could even use the word bona which is the equivalent of 'sexy', but this may seem a little forward. Another useful word is 'carina' which means cute. Telling a girl that she's 'molto carina' or 'very cute' is a sure winner!