Socialization is vital to the mental and physical development of dogs. Socialization should begin in the birth home of the puppy. Good breeders will introduce puppies to stable older dogs so that they begin to learn appropriate dog behaviour. Of course a huge part of their social development occurs with their mom and their litter mates. This socialization should be continued in their new homes and continue for the life of the dog.
However the period of time from about 8 weeks when your puppy comes home to 5 months is a critical period of time for your puppy. It is absolutely imperative that owners take advantage of this tiny window of opportunity to provide the social experiences that their dog is going to need for the rest of its life. Failure to do so will result in dogs that are difficult to live with at best.
Depending on your breed of dog and how it was raised, puppies go through fear periods when potentially threatening experiences can have profound effects on your puppy.
But perhaps doing nothing with your puppy can be even more devastating for the future well being of your dog.
Puppies need to have as much exposure as possible to new and safe experiences. Take them to as many places as you can think of. Most puppies can fit in a smallish crate in even the smallest of cars. Take them to clean areas to avoid the risk of any infection before they have completed all their vaccinations. I take my puppies in a canvas shopping bag to ensure they are not touching the floor and to prevent any accidents. This only works for a short period of time as they increase in weight but it makes it possible for them to visit many places. My bank tellers love it when I walk in with a new pup in my bag. As they get older and have had their shots take them down busy streets, into pet food stores and in clean and safe parks where they can meet new people. Take them in elevators and on stairs. Take them to fields and forest trails. Let them experience life.
Introduce your puppy to safe, puppy friendly older dogs. If you know of someone that has a kind and gentle puppy, that will work too but beware of puppy bullies. Intervene in any rough play immediately if you feel that your pup is not happy or showing any signs of fear at all.
Once your pup reaches the ripe old age of 5 months the main period of socialization is over for good. You can still socialize your dog but it will require a far greater effort for very small gains.