As Rangiroa has only two very sleepy towns - Avatoru and Tiputa - concentrated on a 10 km long stretch of islets in the northern part of the atoll, your choices for getting around aren't too complicated.
Upon arrival at the airport, your host generally will meet you with a car at the airport. If you have a tour scheduled for the day, the tour operator will pick you up at your hotel/resort/pension.
There are several water taxis that will take you from the Avatoru islets to Tiputa village. They allow you to take bikes across with you, so this is a good way to see more of the atoll. They advertise 1000 CFP for a trip across, 1500 CFP with a bike, but they sometimes waive the 500 CFP bike surcharge.
Most of the resorts and pensions have bicycles for rent or loan, and this frankly is all you really need once you have made it to your resort. For the lazy, Europcar has gasoline-powered open air "fun cars" and compact cars available for rent. Several other locations such as the Kia Ora resort and a couple of roadside stands have scooters as well.
Getting to locations other than the northern strip requires a boat. Tour operators run trips to the Blue Lagoon (Lagon Bleu) and the Pink Sands (Les Sables Roses) on the southern side of the atoll.