Proper socialization is essential to a happy dog. As a puppy, after all required vaccinations have been given, the dog should be introduced to as many new people, dogs and locations as possible. Doing this helps the dog be less afraid of new situations when they occur later in life. A socialized dog is more likely to have a lower level of aggression towards strangers (both human and canine).
Ask your veterinarian when it is safe for your puppy to interact with other dogs. It is safe for him to play with humans and probably kittens before his vaccinations are complete.
Ask your veterinarian about puppy classes, where multiple clients bring puppies and children to play together, learn simple commands, and be exposed to strange objects.
Regularly handling your puppies paws, rubbing his belly, trimming his nails (have your vet show you how to do this safely), and looking in his mouth will help your puppy be less fearful when he goes to the vet for a check-up. This applies to adult dogs adopted from shelters as well.