Need Suggestions for Itinerary

My suggest is to read some guide books and get info from the net then generate a rough itinerary and post it here for feedback. Specific questions generally (IMHO) get better responses than broad unfocused issues.

Maybe fly into Pisa or Milan, train to CT (1.5 hrs from Pisa, 3 hrs from Milan). Fly home from Naples.

Stay 4 nights in CT, train to Siena (about 3-4 hrs), OR rent a car from La Spezia and make the drive yourself (about 2-3 hr drive). You'll probably want a car in Tuscany to explore all the hill towns. I'd pick a home base here (Siena/Montepulciano or any of the hill towns most appealing to you) day trip to wherever. Maybe drop the car off here, and then train down to the coast.

4 nights in Tuscany, drive or train to Sorrento (4 - 5 hr via train into Naples, then hop onto the Circumvesuviana from Naples into Sorrento). Possibly make home-base in Sorrento and explore Amalfi Coast, etc. from here.

4 nights in Sorrento, Circumvesuviana back to Naples.

Spend your last night in Naples, and fly back home from Naples.

As mentioned above, a lot really depends on your arrival and departure arrangements.
Pisa, or Milan for arrival.
Naples, or Rome for departure.
There are 7 daily direct train services from Milan to Monterosso in the Cinque Terre, so you could perhaps head there directly on arrival.
To Siena and you will need to change trains at Pisa and Empoli.
It's about an hour directly by train from Siena to Montepulciano.
A car would be very useful in Tuscany, but not a complete necessity. It's easy by bus to San Gimignano from Siena, for example.
You will most probably have train changes in Chiusi, Rome, Naples if you are heading to Sorrento for the Amalfi Coast.

When are you planning to go?

Thanks for the suggestions! We are planning an early October vacation.

Oooh, sounds like a great time to be there. Milan-Rome is probably going to be the easiest route for flights, the 2 biggest airports with the most direct flights to the U.S. though Pisa is more convenient for the CT.

It may be worthwhile to plan spending your first night in Milan (there are sights to see). If you have a morning flight home, you'll want to spend your last night near the airport, so it doesn't matter much if you're flying back from Naples or Rome.

How you plan your ground itinerary depends on how much time you want to spend in each area (which depends on what you want to see and do in each), how many times you want to change hotels and whether you may rent a car for part of the trip.