If you are not under arrest, you have the right to calmly leave.
You have the right to pull over in a safe, well-lit area. If you are uncomfortable pulling over on the highway, call 911 and ask the dispatcher to let the officer know you are looking for a safe area to pull over. Put on your hazard lights to acknowledge the officer.
Common examples of probable cause include the sight or smell of contraband in plain view or plain smell, or an admission of guilt for a specific crime. The presentation of any of these facts would allow an officer to perform a search and make an arrest.
"You have the right to remain silent. Anything you say can and will be used against you in a court of law. You have the right to an attorney. If you can not afford one, one will be appointed to you. Do you understand these rights as I have read them to you? With these rights in mind, do you wish to speak with me?"
First, make sure they are an actual law enforcement official. Ask to see credentials, and if you are still uncomfortable, call 911 and ask if an officer is currently dispatched to your address.