There is no airport at Ribeira Grande. Almost everyone arrives by road from João Paulo II Airport (PDL) in Ponta Delgada, fifteen kilometres south across the island. Direct flights to PDL run year-round from Lisbon, Porto, and Madeira (TAP and SATA), seasonally from several mainland European cities, and from Boston and the US East Coast on Azores Airlines.
From the airport, a rental car is by far the most practical option — the drive over the central ridge to Ribeira Grande takes about twenty-five minutes, and you will want a car for everything outside town anyway. The major rental agencies have desks in the arrivals hall. A taxi to Ribeira Grande costs around €25–€30.
If you do not want to drive, the Linha de Costa Norte bus run by Auto Viação Micaelense connects Ponta Delgada with Ribeira Grande several times a day (about 45 minutes, a couple of euros). It is fine for getting between the two towns but useless for sights spread along the north coast — assume you will need to combine bus, taxi, and walking, or just rent a car.