When you hit a dog with a car you should try to pull off the road if safe to do so and then go check on the dog to see if the dog is okay or not.
If the dog was hit near a house you should go up and see if the owner of the dog is around and let them know what happened.
You're legally required to report the accident involving the dog to the police so do call the police departments number and make a report on it, especially if you cannot make contact with the dogs owner.