From Frommers:
Frequent trains connect Naples with the rest of Italy. One or two trains per hour arrive from Rome, taking 2 1/2 hours and costing from 10� ($13) one-way.
The city has two main rail terminals: Stazione Centrale, at Piazza Garibaldi, and Stazione Mergellina, at Piazza Piedigrotta. Most travelers will arrive at Stazione Central.
The Circumvesuviana Railway (tel. 081-7722444) departs Naples every half-hour from Piazza Garibaldi. However, be sure you get on the train headed toward Sorrento and get off at Pompeii/Scavi (scavi means "ruins"). If you get on the Pompeii train, you'll end up in the town of Pompeii and have to transfer there to the other train to get to the ruins. A round-trip ticket costs 4.60� ($6); trip time is 45 minutes each way.
The ruins are open daily from 8:30am to 5pm (last admission at 3:30pm)
Admission is 10� ($12).
After you pay for your entrance, you'll find a bookstore, where you can purchase guides to the ruins (available in English and complete with detailed photos) that will help you understand what you're seeing. We highly recommend that you purchase one before you set out.