First and foremost you need to ask yourself why you want a dog. Is it for companionship, exhibition or show ring, protection or security, working or trialling, sport and recreation, or a pet for the family?
You need to decide whether you want a small, medium of large dog, quiet or active, long coated or short coated, male or female, and the type of activities in which you and your family would like to participate.
The size and location of your premises will have a bearing on the size of dog you choose. Also bear in mind many small dogs have just as high an activity level as larger ones; they just don't need as much space!
There are many different types of coat, all need grooming and almost all will drop hair. If you don't want an adult with long, flowing coat that requires daily grooming, then buy a short-coated breed!
Males are usually a little larger and stronger than females. Make sure the size and strength level of your dog matches that of your children. If you are not planning to show or breed, spaying or neutering is recommended. A bitch comes into season at least once, or twice a year and must be confined and kept away from males for some weeks. Many health risks for a bitch are minimised once spayed.
As you learn about the different breeds, remember the purpose for which a particular breed was developed, for example a Working dog or a Gundog, active in mind and body, will require regular occupation and a lot of exercise.