First, it is wise to document that the other person is at fault. You should take photographs of your damaged car and show where any physical damage has happened, whether it be inside or outside the car. Make sure you are taking both close-ups and wide shots to show that this is your vehicle.
Try to take photographs of the other vehicle involved. If there is any physical evidence left at the scene, like skid marks, take a picture of those while they are still fresh. Additionally, if you have any noticeable injuries to your body, document those injuries with photographs, too. Get the names and contact information of any witnesses, as they may not appear on the accident report. It's also a good idea to write down all the details of how the accident happened while it is fresh in your memory. You never know when you will have to refer back to it.