Beware of other cars. Talk gently to the dog as you approach. Move slowly and avoid making sudden movements. Put a lead on if possible and, if necessary, muzzle before handling. If your dog can walk, go to the vet, even if there appears to be no pain. There may be internal injuries that are not immediately obvious.
If the dog cannot walk, small dogs can be picked up by placing one hand at the front of the chest and the other under the hindquarters. Improvise a stretcher for larger dogs with a coat or a blanket. If the dog is paralysed, there may be a spinal injury, so try to find something rigid, such as a board. Slide the patient gently on to this if possible. Cover with a blanket to reduce heat loss.