The easiest way to reach Pompeii from Naples is the Circumvesuviana railway. Trains depart from Naples Piazza Garibaldi (Stazione Centrale) every 30 minutes and take about 35 minutes. Get off at Pompei Scavi – Villa dei Misteri — this station is right at the main entrance to the archaeological site. Note the spelling: the modern town is "Pompei" (one 'i'); the ancient city is usually written "Pompeii."
The same Circumvesuviana line connects Sorrento to Pompeii in about 30 minutes. It also stops at Herculaneum (Ercolano Scavi) on the way from Naples.
By car from Naples, take the A3 motorway to the Pompeii exit. Paid car parks are available near the excavations.