What should I do?
You should contact your automobile insurer immediately if you havent done so already. This is why you carry automobile insurance. You need to contact them, let them know that a claim for damages has been made against you arising out of operation of the automobile, give them all the facts, names, police information, etc. They will then investigate and determine if you are liable and pay the damages applicable up to your policy limits. But, most insurance contracts require you to notify them of an accident claim as soon as practicable, so you need to contact them immediately. If you fail to timely notify them, they may deny the claim for breach of terms of the policy.
If you have already contacted them, then you need to turn whatever you received in the mail over to your insurance company and let them handle responding to it.
Best of luck.