We are planning our Sept 2015 trip. We will be staying in Volterra, Rome and Sorrento. What order should we stay in these places and what airports?
For Volterra you could use either Pisa or Florence to fly into, and for Sorrento you could use Rome or Naples. Although really, train connections can get you just about anywhere so even if you don't have the most ideal flights it's not a big deal if you end up flying round trip into Rome. BTW, you will either need a car to get to Volterra or will have to take a bus.
I would prefer not to rent a car. Where can I get the bus for Volterra?
We loved Volterra. Car would be best but buses from Florence, Siena, Pisa and San gimignano. Get rick's Italy book and you will have lots of information.
The first thing I do is nail down the flights, which you will not be able to even start for another month. You can set up flight alerts and also investigate alternative airports near your home. Once your flights are set, the order of your visits will be fairly obvious and you can nail down where you are staying and lastly the local transportation. There is often a wide variation in the airfare by just changing the departures by a few days.