To be a cop in GTA 5 RP, you will need to join a server that has a police role available. Many servers require players to apply for the police role, as it is often a position of responsibility and requires a certain level of role-playing skill. Once accepted, you will be able to create a police officer character and participate in police activities such as patrolling, arresting criminals, and investigating crimes. Keep in mind that each server may have its own process and rules for becoming a police officer, so be sure to read and follow the server's guidelines.