The privately operated Naxos Tourist Information Center (tel. 22850/22-993; fax 22850/25-200), across the plaza from the ferry pier, is the most reliable source of information and help. (Don't confuse it with the small office on the pier itself, which is often closed.) The center, run by the owner of the Chateau Zevgoli provides ferry information; books charter flights between various European airports and Athens, accommodations, cars, and mopeds; arranges excursions; sells maps; exchanges money; holds luggage; assists with phone calls; and provides 2-hour laundry service.
Naxos has a helpful website, www.naxosnow.com, with maps and a photo tour of the island.
If you enjoy map reading, get a good map as soon as possible, as Hora (Naxos town) is old, large, and complex, with a permanent population of more than 3,000. The free Summer Naxos magazine has the best map of the city. Map or no map, prepare to get lost at least once in the little streets in and around the kastro. The Harms-Verlag Naxos is the best map of the island, but it's pricey at 7€. John Freely's Naxos (1976) remains delightful and helpful.
Commercial Bank, on the paralia, has an ATM. It and other banks are open Monday through Thursday from 8am to 2pm, Friday from 8am to 1:30pm. Naxos has a good 24-hour health center (tel. 22850/23-333) just outside Hora on the left off Papavasiliou, the main street off the port. The police (tel. 22850/22-100) are beyond Protodikiou Square, by the Galaxy Hotel. The telephone office (OTE) is at the port's south end; summer hours are daily from 7:30am to 2pm. The post office is south of the OTE by the basketball court; it's opposite the court on the left, on the second floor (Mon-Fri 8am-2pm).