Taxis are easy to get at Hanoi airport. There is a queue just outside the terminal, at roughly the center of the drive. As at all international airports, you should avoid any touts who approach you in the terminal building. Taxis are metered, with the cost of transport to a city center hotel running about 200,000 Dong (-200,000.00 USD) for the 45 minute trip.
If you want to save yourself some hassles, you can book your airport transfers in advance through Viator. The following services are available: