If it's your first time out there, I would probably suggest Rome, Florence/Sienna or Venice.
I'd be very tempted to say Venice. I MUCH prefer it in the winter (and I am assuming you're going in the winter) to the summer. It's less crowded and the colder weather suits the place more than the hot summer.
Having said that Rome is always lovely and Florence would benefit from the lack of tourists that spoil it in the summer. If you go to Florence you could also spend a day in Sienna which is absolutely wonderful.
Of course if you want something a little grittier, you could try Naples - but it's something of an acquired taste whereas you can't go wrong with more northern areas.
Hope that's given you some ideas!
lol watch out for dirty pervy Italian men, they're everywhere and aren't subtle.
I'm Italian and live 60 km far from Venice! I highly recommend it!
I have visited all the cities you've mentioned, therefore I can answer your questions about my fab country!
I would reccomend Rome- for its sheer beauty and uniqueness.....Tuscany, Florence and Siena. Having been to these places i can hardly reccommend them enough!
Rome was brilliant esp as i went with my frinds....it was gorgeous...the food was brilliant too
TO be honest, wherever you go u wil have a brilliant time.
Also, Sardinia has beautiful beaches and is gorgeous.
ps. It is true about the italian men lol
wherever you go ,have fun!