Naples airport is close to the city center (about 15 minutes), but I doubt you want to walk the distance, so you have a few options. The easiest one is the shuttle bus (Alibus) which costs 5 EUR and will take you to Garibaldi Square or the port of Naples. The stop is 100 meters in front of the airport, and you can get tickets from the driver or a machine inside the arrivals terminal. You can also use a public bus numbered 3s for 1.50 EUR, but it's way slower and more crowded, or you can get a taxi for 16 to 20 EUR. If Naples is not your final destination, but you're going straight to Pompeii, Sorrento or Positano, here you can find transfers directly from the airport.