How do you file charges of violating law agenst a police officer?

Depending on what you mean by law? If a state statute was violated by a law enforcement officer and you want to report it the first thing I would do is contact the law enforcement officer's agency where he or she is employed by. Contact there Internal Affairs department and report the actions to them so it can be investigated. Sometimes police officers live by a code of brotherhood so your next step would be to contact your state's law enforcement agency. Here in Florida it would be the Florida Department of Law enforcement.