Its not usual to cut them back at all, but can you add a photo if you don't know which bamboo you've got?

As Bamboo says, but if you really need to cut back, follow these steps:
Determine a target height that is compatible with your bamboo species--it is difficult to keep a bamboo less than 2/3 its normal mature height.
It is getting late in the year--if you are in the UK--so if you want to get a start, cut just above side branches, don't use hedge shears, and don't cut off more than 1/4 of the foliage. In milder climates, more could be removed, but 2/3 of the foliage should remain.
In spring, as new growth emerges, you can remove a little more height, and keep an eye on the new shoots from the ground. When they start to produce branches, cut them off just above a side branch, about 2-3 short of the target height. The arrangement of branches will adapt to produce a more natural looking cane and plant outline.