We’re in Florence now & heading to Sorrento tomorrow. We booked our train to Naples but we’re having trouble getting the train from Naples to Sorrento. What’s the secret? Can we just buy them in Naples?
The train from Naples to Sorrento is just a local commuter train. Basically like a subway. Buy tickets at the station and hop on the next train. Follow the signs for the Circumvesuviana in Naples.
As already mentioned, the Circumvesuviana is a separate company from Trenitalia. It is quite easy, the Circumvesuviana is underneath the Napoli Centrale station, and well marked. The trains themselves are old and scruffy.
There is a Campania express, which is 30 minutes faster as it stops at fewer stations - but otherwise it is little different except for a higher cost.
You might find this link useful
One small addition:
Buy tickets at the station,, and validate them before hopping on the next train.