If using Capodichino, of several ways to get to places such as Pompei and Sorrento (either as a destination in itself, or to then continue on to Positano, Amalfi etc), one of the simplest and most economical is on board the buses of the private Curreri Viaggi line, whose website is here....
Click on the Union Jack symbol for an English version, and then on the "Bus routes" tab.
A few notes:
1. To either destination, oneway tickets - which you purchase from the driver - are currently €10. First come, first served - with no advance bookings accepted!
2. The journey time to Sorrento is scheduled for 75 minutes, but can vary with the traffic.
3. The site shows year-round daily departures from Sorrento at:
06:30, 08:30, 09:00, 10:30, 12:00, 14;00, 15:00 and 16:30
... and from the airport (the stop is only yards from the Arrivals building exit) at:
09:00, 11:00, 12:00, 13:00, 14:30, 16:30, 18:00 and 19:30
In Sorrento the stop is near the town's Circumvesuviana railway station - and handy, for instance, if continuing on by SITA bus to the villages of the Amalfi Coast.
See the website for details of stops in other resorts along the way. For some you may have to tell the driver on boarding - so he knows to take an appropriate route!
If in doubt, email them using the form on the "Contacts" tab, or phone on 081.80.15.420 or 081.80.16.376
PS.... A slight oddity: the site shows one can go from the airport to Pompei, but offers no service for the return journey - so you'd need to find an alternative!