Ha Long Bay is the premier tourist destination in northern Vietnam. The vast majority of visitors come on organized tours from Hanoi, travelling by road to Ha Long City, it takes about 3,5hours to 4 hours, on the bay's northern shore, where they board replica wooden junks to spend a day or two days cruising or more on the bay. While an organized tour is without doubt the easiest option, it's perfectly possible – though not necessarily any cheaper – to do it under your own steam. Both Ha Long City and Cat Ba, the other main jumping-off point, on the southern edge of the bay, can easily be reached by public transport. There are frequent bus services running out of Hanoi to Ha Long City, while Cat Ba is served by a combination of train, bus and boat from Hanoi via Haiphong.
It would be better to travel long jouney by travel agent, it really something if you are experienced by local transportation like buses or slow train.