Socializing your dog should be a life long process. You want your dog to be exposed to as many people, pets and situations as possible so he learns how to behave and react to different things. Take him to new places every few days or so and introduce him to family and friends.
Introducing To Other Pets
If your dog has been socialized growing up, he shouldn't have problems meeting other dogs. But you still want to take it slow and see how things go before letting him loose. The best way to start is by introducing your do to other dogs that you know are friendly and socialized. When introducing your dog to new dog friends, keep him on his leash so he can sniff the other dog, and you can reel him in if you need to. Try to keep the leash slack; if you pull too hard, you can trigger his "fight or flight" instinct.