We arrive at FCO but want to travel immediately to Sorrento to begin our tour. What is the best way to get from the airport to Sorrento?
Leonardo Express from FCO to Roma Termini (every half hour, travel time about an hour, validate your ticket), fast train to Napoli Centrale (trains leave Rome every half hour, travel time a little more than an hour), go downstairs to the Circumvesuviana station, Sorrento is the last stop on the line (trains leave every half hour, travel time a little more than one hour). You can buy the LE and Rome-Naples tickets at FCO.
Have you considered booking through or connecting by plane to Naples? You could then take the Curreri bus service from the Naples airport to Sorrento. Short and sweet!