While Out Walking On A Leash
When meeting a new person, first determine your dog's body language. Try to determine if he is friendly, fearful or aggressive. These emotions may alternate quickly, so just try to determine the one most prevalent. If they change, then you change your response appropriately. Praise friendly, but temporarily move the pet away if acting fearful or aggressive, and follow the instructions below.

Try to determine if he is friendly, fearful or aggressive. These emotions may alternate quickly, so just try to determine the one most prevalent.

Friendly Is Best
If acting friendly, your biggest concern is that your dog shows proper manners, and doesn't jump on, slurp, claw, crotch sniff, or otherwise act like a nuisance. The goal is good canine citizenship. The way to teach proper manners is to give lots of praise for correct behavior, and if incorrect, pull the dog away, make him sit, then repeat the greeting attempt immediately. If the other person is willing, this repetition is a great teaching opportunity. It is recommended you tell the other person up front that you are teaching your dog proper greeting manners, and ask their indulgence to only pet the dog if he is acting properly, and otherwise turn and ignore him.

Treat Fearful And Aggressive Response Similarly
If your dog acts either fearful or aggressive, this is inappropriate, and will limit his options for outings throughout life, so the recommendation is to do something about it now. Pick a Instruction that communicates to your dog that this person is non-threatening (e.g. "Say Hi"). Use this phrase consistently when meeting a new person and want your dog to act friendly.

First, start at a distance to determine the reactive range. Do not totally avoid the target person, unless you want to anyway. Leaving reinforces the dog for being reactive, and makes it slightly worse for next time. The ideal scenario, is to find that distance from the person (e.g. if they are sitting on a bench) where your dog stops acting reactive. At this distance begin to either play with your dog, or give Instructions and treats. As you work with the dog, gradually move closer to the new person. Praise any relaxed, happy posture and attitude, and if the bad attitude returns, ignore it and move farther away. The goal is reduce the reactive distance.

When you are ready to actually approach a person close enough to talk, explain that you are teaching your dog proper greeting manners, and ask it they would mind helping for a few seconds. You should be aware that if you force any situation, your dog might become so stressed as to bite, so don't try to do too much at one meeting. It is up to you to protect the other person from injury. Keep your dog on the leash but greet the person, shake hands, act relaxed, and talk to your dog in a high happy tone explaining that this is a nice person.

One suggestion is to step next to the person and turn so you and the person are standing shoulder to shoulder facing your dog. This body language communicates that you and the person are, "on the same team" and your dog becomes the odd one out. Your dog should immediately accept anyone you place "on your team."

Take your dog on walks when he is hungry and bring along a few treats (plus a baggie to pick up after your dog). Use these treats to reward relaxed, friendly postures and movements when meeting strangers. Have patience. Depending on how long your dog has been acting this way, it may take many months, and dozens of practice encounters before he can meet new people with proper canine manners.