What to study to become Detective?

What path in studies and all should I pursue if I plan to become a detective(like in law enforcement, probably like CID/CBI/FBI, etc.), (without being a patrol officer and all first, and then taking the long-term journey?)

Also, will the same path work out for an investigative psychologist? If not, then what path should I pursue in this case? Can I work in(or with) law enforcement in this case?