Socializing your dog is one of the most important things you can do for his or her behavior. It is ideal to introduce your pet to other dogs as soon as they are old enough to have all of their shots. Unfortunately not all dogs have that opportunity from the start. For instance, rescues sometimes get raised in a neglegent or abusive enviroment and grow up to fear other animals. This can sometimes turn into aggressive behavior. If you want your pet to be pet and family friendly you must socialize your dog just as you would a child and here is how.
Walking your dog regualarly gives your four-legged friend the opportunity to meet and greet with other dogs and people. During the walk your dog is burning off energy which is a far better state for them to be in while interacting with other dogs and people. Keep in mind, some dogs can be leash aggressive when they see other animals or humans. Remember to never yell at your dog or pull back on the leash, this adds excitment and negative energy to an already difficult situation. As Ceaser says "Stay calm and assertive" and distract them with a correction instead of a negative response.
If you know that your dog has aggressive tendencies or growels when another dog approaches them it may be a good idea to have your dog wear a muzzel. I know what your thinking "that's so cruel or embarassing". Think of it this way, if you know your dog is aggressive you may refrain from taking your dog out for socialization. If that is the case the dog will never make any progress and you will never be able to bring your dog places. When your dog is wearing a muzzle they become calmer and more receptive to meeting and make it a positive experience for everyone.
For more information check out "How to Socialize an Adult Dog" by Josh Weiss-Roessler on www.cesarsway.com.