Probably none, anyone with half a brain can do it, it isn't necessary to be very clever, you will probably not be very well liked, it is a job for people who are abit on the snide side and want to make peoples lives hell i think. Work in a bakery or something instead, more fun
You want to be a Private Investigator, and you couldn't work out how to find out how to start. Do you really think you are suited for this kind of work?
If you are still interested a good starting point might be Google!!!!!