You have two options when this happens.
You can contact your insurance agent and tell them what happened. This is a good option if there is damage to your car that you want repaired. You will most likely be prompted by your agent to file a claim and will be responsible for the damages. Get an estimate of the damage and determine if it is greater than your collision deductible. If it is and you do want the damages repaired this is a reasonable option. Depending on the company you are with you are only allowed so many claims within a certain period of time. So before you file a claim talk to your agent about your deductibles and what repercussions there will be if you file a claim.
Your other option is to not tell your agent and wait to see if the other person files a claim on your behalf. If you are at fault and the only damage done was to the vehicles this is not an unreasonable option. The dangers of this option are the other person may file a claim and report bodily injury. This may include sore neck or back that requires a doctor’s attention. Remember to write everything down as you remember it happening and take pictures of the damage. This information helps later on if a claim is made by you or the other party.