According to the Humane Society, the best thing to do with a lost dog is take it to the animal shelter. Another option is taking the dog home and trying to find the owner. However, it is best to call the animal shelter and notify them of the lost dog.
When a lost dog is found, the first thing to do is look for the dog's tags. Typically, tags have rabies information as well as the address and phone number of the dog's owner. If the dog has tags, get the dog to safety and call the owner. In the event that a dog does not have tags, take the dog to the animal shelter and see if it has a microchip. If the dog has no tag, try posting notices on local pet websites and in the newspaper.