Clean and disinfect the skin. You can apply Neosporin or a similar antibiotic cream. Take the puppy to the vet. He may need stitches or some other treatment.
You should definitely take your puppy to a veterinarian. Only a veterinarian is qualified to diagnose the severity of the tear and prevent an infection.
If it's bad enough, you should see your vet immediately. For a real small tear, clean it well with peroxide/alcohol and put it back together with super glue.
