Socializing your canine companion with people and other canines is very important. It is best to start socializing as young as possible. In doing so, it can help prevent certain aggression problems later on.
A friend of mine Jen; has owned her dog Diesel, since he was 11 weeks old and he is now 12 years of age. Diesel was socialized as a pup but only to a few other dogs, so as he got older he showed more aggression towards dogs he was unfamiliar with. Diesel also had some aggression towards people, but only when they were walking or moving away from him. He would be the dog to bite you on your way out the door not as you were coming in. When Diesel would act afraid of something; say a sign on the side of the road, Jen would pick him up, instead of encouraging him to approach the sign. Now, in Diesel's mind, the sign is something to be afraid of. Think of a child falling down and scraping its knee, the bigger the fuss, the more the knee hurts, the more the child cries.
When Diesel was about 7, I introduced Jen to Mark, a friend and dog trainer who has helped me out a lot in the past. Mark started working with Jen and Diesel by having Jen join his dog walking club so she could start socializing Diesel with other dogs and people. Jen started out on the walks about 20 feet behind the group, but within a few weeks she was walking side by side with a select few. Within a couple of years Jen and Diesel were walking with everyone and every dog in the group. People in the dog walking in the club can now walk up to pet Diesel and he wants them to pet him instead of trying to bite them. She can even take him to the dog park now.
If Jen had started socializing Diesel at an earlier age the aggression more than likely would not have developed so strong. I say this do to the fact that Diesel has changed and can be with other dogs now and wants to be with other dogs. Instead of wanting to bite them or attack them he wants to play with them. Instead of trying to bite people, he now wants people to pet him. Diesel actually wants to go up to people as he walks by them now, where before he was ready to lunge and attack. I am not saying that Diesel is never going to show aggression, that potential is there, but it is more manageable now and more controlled on both Diesel's part and Jen's part.
Take the time to socialize your companion and allow your companion to be the best companion he/ she can be. Take advantage of every opportunity you can in socializing your new companion. Take them to the park, or the pet store; since most pet stores allow you to bring your pet in these days and just walk around encouraging your companion to meet and greet the proper way. Joining a dog walking club is another great option, if there is one nearby.