Have you tried to search the web for the needed information?
For example if you enter: sd+customer+hierarchy site:sap.com in most of the popular search engines, it produces a lot of results. The documentation in help.sap.com in particular for customer hierarchies is very detailed - I believe you will find it useful for understanding the concept.
In case you have faced a specific problem with the setup or processing - please explain what is the issue, so that the forum members could assist you in the investigaion.