How to socialize an adult dog? Meeting with an adult dog should be calm and friendly, no matter whether the dog is a pooch or a pure-bred dog, small or big dog. First of all, define, how does a dog reacts on you. 
Everything should be peaceful, without aggression or evil. Then go on to get acquainted.
Coming closer do not try to pet the dog immediately. The dog may even not notice you and turn back to its place in a crate. It is normal for the first stage of getting used to you. But only if there is no aggression or panic. Try to sit down and call a dog's name showing a toy or a treat. The best way is when a dog comes itself. Wait until the dog waives its tail in response to your friendly words showing that you are not dangerous. Do not even try to caress it until it shows friendly signs, waving tail etc. Move an open hand forward to the dog. Let a dog come to smell it himself. After that try to caress a dog, but not up on its head, but below and by the side of its jaws.
If a dog reacts with aggression, cannot keep calm, does not want to get in contact with you, do not give up the thought 
of rehoming this dog, even if its owner tells that usually the dog behaves another way.
Each dog is self-esteem and unique. But there are common variants of dog behavior.
A coward dog lays down to the ground and presses down ears and tail at meeting, whines, tries to jump aside or run away. Such a kind of dog is difficult to socialize, adapt. It is afraid of noises, people, strangers, other animals and strange things. If you are lucky, a dog will become calmer with time, but the cowardice will appear in a stressful situation again.
A cowardly-aggressive dog has all the features mentioned previously, but sometimes it attacks, barks aloud, and the next moment jumps backwards or runs away. Int he state of panic this kind of dog may be absolutely uncontrolled and injure a weak or a frightened person. If someone will answer 
a dog aggression with strength, it will run away whining.
A dominantly-aggressive dog may attack without warning. It often fights with other dogs, may bite a strange person in walking. Such a dog is difficult to train and care. He will not react and follow your rules. The only way to train a dominant aggressive dog is to get the help of a trainer-behaviorist or a canine psychologist. But better is not give up rehoming an aggressive dog.
A psychically normal dog will not notice you at once or will treat you calmly: come up, smell, let caress it. There are no troubles expected with such dog.
If you are rehoming a dog from the previous owner, ask him to give you all dog accessories, bowls, toys and even bed. Then it will be easier for the dog to get used to the new owner and home. If you decide to get a dog from the street - care of all the needed tools for its keeping.
Your task is to establish definite rules at once and follow them strictly. It will help the pet to treat you as a leader and take the required position in the family.
These tools will help you to train a dog and to make dog walking easier and pleasant:
More dog accessories for your pet:
Leather dog collars
Leather dog leads
Dog toys
Dog care accessories