Ask for a lawyer immediately.
Don't sign anything.
Don't agree to anything without a lawyer present.
If you call a lawyer, the police are not allowed to listen. If you call anyone else, they are likely to listen.
If you do explicitly consent, it can affect you later in court.
Visual records are fully protected, but some states have tried to regulate the audio portion of videos under wiretapping laws.
Get contact information for witnesses and take photographs of any injuries.