The only one I have direct knowledge of is the FBI. You are being much too general when you ask. If you want to be a special agent, pretty tough standards. If you want to be a filing clerk for the FBI, not so tough. I assume you are interested in doing something like FBI Special Agent. If so, you need a degree, must pass extensive background check, etc. Must have working experience before you will be considered for position as special agent. They do not accept new college grads without experience into special agent program. Also, the application is like a damn book. Good Luck.