Anita,
If you're planning to travel this September, you'll need to get cracking and get some bookings made! Does your 12-14 days include flight days? You'll arrive in Italy the day after you depart your home airport, and if you're like many of us you'll be jet lagged for the first few days so won't be up to full "touring speed".
As this is your first trip to Italy, I would highly recommend picking up a copy of the RS Italy guidebook, as there's an enormous amount of information there that will help your trip go smoothly. If this is also your first trip to Europe, it would be a good idea to browse Europe Through The Back Door as that provides good information on how to travel well in Europe.
For an Itinerary, my suggestion is along the same lines as what Roberto suggested. In mid-to-late September, it would probably be a good idea to start in the north as the weather will be a bit warmer, and work south from there.
There are many possibilities, and you could consider something like this.....
- Flight inbound Milan
- Arrive MXP, train to Varenna (Lake Como) - 2 nights (Varenna is a perfect place to get over jet lag, and get used to being "in" Italy - it will provide a more gentle introduction than starting in a large city).
- Train to Florence - 5 nights (use Florence as a home base to explore Tuscany or perhaps even a day trip to the Cinque Terre - Siena is a good day trip or you could even stay there for a night or two on the way to Rome).
- Train to Rome - 5 nights (again use Rome as a home base, with day trips to Ovieto which is easy, or even to Naples or Sorrento).
- Flight outbound from Rome
This arrangement minimizes changing of locations and hotels, which have a cost in both time and money, and should allow you to have a good introductory trip to Italy. As Rick says, "assume you will return". I'd suggest leaving the Amalfi Coast and the south for another visit, when you'll have more time to explore it properly.
As this is your first trip to Italy, there are some potentially expensive "caveats" to be aware of when using trains and other public transit in Italy. If you need more information, post another note here.