Located on high mountain with no airport, Sapa is accessible by road only. From Hanoi you have three way go to Sapa: By bus, by train and private car.

1, By train.
Traveling by train takes about 8-9 hours and the train only stops in Lao Cai, the province which Sapa belongs to. It then takes another hour to get to Sapa from Lao Cai station.
2, By bus.
There are daily bus departing Hanoi at 6:30am or 10:00pm. One way trip takes betwen 6 and 7:00 hours.
3, By private car.
This is the fastest and most expensive way to go. Time for transfer is 5 hours (depend on your accommodation in Sapa).