Hi there !
Ha long City is 170 km away from Hanoi in a good road condition, where you will take the cruise out to see Ha long Bay. The best way to travel to Ha long bay is to rent or hire Private Car, Mini Van (Minibus) or Bus with a driver, the drive takes approx 3,5 hours one way, cost about US$80 to 120 one way/trip and US$ 150 to 200 for return trip depend on the type of vehicle. The driver also can pick up or collect you at the airport if you want to hire your own car or bus.
If from airport you can check the price Noibai Taxi the web :
http://www.noibaitaxi.com
Hope this hefp
WesHk