If you have a rental car you will enjoy your stay in a small town like Reggello much more. The closest train stations are at Figline or Sant'Ellero, which are not major stations. To reach them you need the ACV buses. With a car you'll have more flexibility and save time to go to places.
If you do rent a car (recommended), the easiest way to Sorrento is to drive there. If, once in Sorrento, you don't want the car anymore, you can return it in Sorrento.
By train is a long trek. First you need to go to Fligline station, then make your way to Rome and then to Naples. In Naples you then take a Circumvesuviana train to Sorrento.