In Hanoi you will find plenty of agencies that offer tours to Halong Bay. Usually they offer one-, two- and three-days trips. One day is definetly not enough to visit the bay. Only the bus ride from Hanoi to Ving Ha Long takes almost five hours! With the one-day excursion you will only be able to do a small boat tour. Two days is the minimum stay I recommend. If you add a third day, you will be able to sleep in one of the islands, Cat Ba Island for instance.
The trip from Hanoi is done by bus. Your guide will bring you to Halong Bay port, where you will get on one of the boats.
♥ Visit Tagulandang Island ♥