It's really important you give us as many details as you can
In particular we need:

Take photos of the scene and any damage to your car, from all angles. We’ll ask for this after you’ve submitted your claim.

Get the details of any other drivers involved: their full name, phone number, address and registration number.

Make a note of where it happened. An approximate address will be helpful.

Get names and phone numbers of anyone who saw the incident.