An officer may arrive on the scene on their own, or you may need to contact them if the situation is more serious. If they do not need to come, then you can call the police to make a report of the incident. When an officer is making a report they will include detailed information and will give their view on what took place. They will want to take your statement and you should comply with their questions. Do not get into who is at fault at this point, as this can be addressed later and you may say something that you do not mean.