In addition to what Steven posted here are a few more things to consider:
How much time will you have to exercise your new dog?Some smaller breed dogs can get by with a short walk, but others need to be walked or ran for an hour or more every day. You need to be honest with yourself and decide what amount of time you are willing or able to spend with your dog. Be sure to consider both, your physical abilities and your time schedule. If you'd like to own an active dog but your job keeps you busy 10-12 hours a day, you may not want to get a really active dog. Active dogs need to go for long walks or runs with you every day, not just on weekends when you don’t have anything better to do. An active dog would be miserable (and probably very destructive) during the work week if you can’t spend the time to exercise him properly.
How much training can you do? Regardless of what kind of dog you decide to adopt, a trained dog will be much easier to live with. A well trained dog can go to more public places with you. This is because they cause less of a disruption. A well trained dog will also be more easily integrated into your life. Obviously a puppy will need more of your attention, while most older dogs have had at least some training and will therefore take less time to train.
Where will the dog live?In the past few years a lot of individuals and professionals have adopted the opinion that dogs who live outside are more apt to be neglected and abused than those who live inside. They feel very strongly that all dogs should live in the house. Although most any dog will do well inside if it is given enough exercise, some dogs (given proper shelter and attention) are also equipped to live outside even in the coldest winter conditions or in the hottest summer conditions. Some perfect examples of this are the Labrador Retriever, the German Shepherd and the Siberian Husky. They all are capable of handling the cold weather, but don’t really do to well in the heat of summer because of their double coat of fur. If your new dog will be spending any time outside, you must consider your area's climate when you are choosing a breed. If your dog must live outside, be sure that it has adequate (enclosed, covered, maybe even heated or cooled) shelter, and make an extra effort to spend time making sure that your dog has enough food, water and shelter during severe hot and cold weather conditions.
How much grooming are you willing to do or pay for?Dogs that have long or curly hair will require you to spend more time to keep their coat free of tangles and mats. These types of breeds will require a lot more time on your part to keep them properly groomed and if you can not do it yourself it will cost you more money to have a dog professionally groomed on a regular basis. Many of these breed types may require regular grooming every 6-8 weeks. Even short haired dogs like the Dalmatian, the Chihuahua and others that are fairly low-maintenance can go through periods of profuse shedding and their coats will need extra attention. No matter what breed you choose, all dogs need to have their nails, eyes, and ears taken care of on a regular basis.
What do you plan to do with your dog?Are you looking for a watchdog, a loyal “fireplace” dog, or a running partner? Maybe you want an active and athletic dog that you can do things like agility training, hiking, herding or hunting. You might even be looking for a dog that you can train for Police Work, Therapy Work or as a Service Dog for you or a loved one. The reason you want or need a dog can really affect your breed choice because, for example, most toy breeds just don't make very good Police dogs.
What kind of past experience do you have with dogs?Everyone has a first dog at some point. After all, we all had to start some where. But it’s important that you know that there are a few breeds that are not recommended for first-time owners. For instance, an older Dachshund is a lot easier to care for than a one year old Husky, a Bull Dog or a Border Collie.
Are there other family members in your home and are they all willing to teach, and care for the dog?Adopting a dog should be a family decision and all family members should be a part of raising the family dog. Teaching children to properly care for and train a dog can make a lasting impression on them. Learning how to control and train a dog can help build self esteem and help them become better with people too.
from a betterdog4u.com