When choosing your puppy, there are several things you need to keep in mind. The size and weight of the dog breed is important as well as many other characteristics, e.g. health, temperament, shredding, exercise, etc. If you want a lap dog, you may need a smaller dog that can sit quietly for long time. Many high energy small dogs don't become a good lap dog while the giant Great Dane is sometimes called the biggest lap dog.
Big dogs may eat a lot and need a lot of exercise but may be good companions and will fit better with your lifestyle than a smaller dog. You maybe want to buy a highly energetic dog or you don't like to be disturbed constantly to play with your dog. On the other hand, low-energy dogs may irritate children for their lack of movement.
Different dog breeds have different exercise requirements. While some dogs need to do tough activities such as playing football and rock climbing, others can be kept fit by merely a stroll in the park. There are dog breeds that do not need much exercise at all, and roaming around in the home is enough for them.
How is your living situation? Maybe you need a dog that is good with small children or you live alone. Some dog breeds doesn't go at all with children while others become your children's best friends. Other things to keep in mind are the strength, boldness, protectiveness and courage of the dog, how often it needs to be groomed and whether the particular dog breed will be able to tolerate the temperature of the place, where it has to live.
One word of advice: There are many different reasons for buying a puppy but as more you prepare yourself and select the puppy that fit you best as more you will enjoy sharing your life with your dog. If it is your first puppy, look for a puppy raised indoors around human companionship and influence - specifically around people who have devoted lots of time to the puppies' education.