Being a cop is a nice job mate! While you may get hated at a little bit, it's Cops and Robbers so what do you expect? Anyway, to become a cop you need to be Level 7+. When you have the required level, go to a police station and talk to the NPC, pass the police exam and switch your job.
Open the category menu to find other guides.
REMEMBER TO READ THE RULES BEFORE PLAYING COP!!!