If your dog is afraid of people, be prepared to protect your dog from well-meaning friends and strangers. Initially the best approach to take with a fearful dog is to put as little pressure as possible on it. This means avoiding direct eye contact, not talking to the dog if it shows discomfort or fear when spoken to and handling the dog as little and as gently as possible. It doesn't matter what your intentions are, your dog will still be afraid. Your goal is to help the dog to be in the presence of people, however peripherally, and not feel fear. Ask people to ignore your dog until it is ready to have strangers interact with it.
If your dog has a specific fear; other dogs, children, thunder, car rides, etc. you will be creating situations or moderating situations so that your dog learns that these things are not only ok, but can also be wonderful (other dogs play, children give treats and car rides take them to the park). You do not want to rush your dog into confrontations with its fears, but at the same time you do not want to isolate your dog so that it never gets to practice being comfortable with the things it fears.
Every dog is unique and each will be in a different place on the spectrum of uncomfortable to horrified in regard to its triggers (the things it fears). As you work with your dog you want to notice it moving down the spectrum (becoming less reactive in regard to it fears as opposed to more). For some dogs it may only take a few sessions for it to learn that something isn't as bad as they thought it was. For other dogs, it can take months to years to change their behavior. You might want to talk to a vet regarding medications that can help your dog through this process. There are also a number of over the counter supplements that some people have had success with.
Working with these dogs is not easy. Prepare to be frustrated and mystified by your dog's behavior. The more you learn the more sense their behavior will make, but if you're like many owners of fearful dogs, you've ended up with more than you bargained for when you got your dog. If you've decided that you're in it for the long haul you might want to meet with a trainer or behaviorist to be sure you're on the right track. If you decide that you are not in the position to work with your dog, be up front and honest with anyone you pass it along to.