Sorrento is very easy to get to.

From Naples main train station (Napoli Centrale) you just go down the stairs and that’s where the Circumvesuviana train is (it is just like a metro and is clearly signposted). This will take you to Sorrento in about an hour and it is approximately 3.50 Euro for a ticket. Make sure you buy it from the guy in the booth and not from the machine opposite because the machine only gives you a ticket for Naples metro not the circumvesuviana. 

The same line takes you to Pompei which is a good place to visit.

If you are staying at Sant A’gnello,then this is also the train you need to get on  because it is a few stops before Sorrento.

Once you arrive, it would recommend that you have a map if you are walking, any map should be good because Sorrento is not complicated. If you would prefer a taxi then in Sant A’gnello there were ones outside the station so it would be the same at Sorrento station. You can easily get a taxi from Piazza Tasso which is the main piazza in Sorrento situated on Corso Italia.

On the Circumvesuviana Italians will probably get on playing an instrument for money - try not to stare.