From Naples Railway Station (P.za Garibaldi)
If you catch the train, please get off at Napoli Centrale. From there you can catch the local train Circumvesuviana to reach Sorrento.

From  Molo Beverello -  Napoli
From Molo Beverello (Napoli, in front of Maschio Angioino)  you can take a boat (aliscafo) and arrive at Marina Piccola of Sorrento.
From the port you can take the yellow  bus of circumvesuviana that connects the port to Piazza Tasso, the central square of Sorrento.