WHICH BREED SHALL I CHOOSE?

Consider what you have to offer the dog, not just what the dog can offer you. If this is your first dog, take a look at this guidance, http://www.getpuppysmart.com/
WILL THE CHILDREN BE SAFE?
Do you have young children? Very young children sometimes harass dogs, so be cautious. There's nothing wrong with big dogs like Rhodesian ridgebacks, Akitas and rottweilers but they have a big bite if things go wrong. Choose a gentle breed. You want a dog that is not over-sensitive, but patient and tolerant.
WILL IT BE A TOWN OR COUNTRY DOG?
Do you walk in the park with other dogs? Chasing breeds like greyhounds and lurchers may be set off by fleeing small animals including small dogs. You should walk them muzzled (nothing wrong with that, incidentally) - just to make sure. Untrained guarding breeds may be difficult around other dogs.
DO I HAVE THE ENERGY?
As a rough rule border collies need two hours exercise or a full hour with a chase toy to double the distance. Clever sheepdogs also need things for their mind. There are suggestions in my book, One Hundred Ways to a Happy Dog. Jack Russells and many other terriers are also tireless. These breeds are not a good idea for elderly people.

WHAT ABOUT THE CAT?
Do you have a cat? Chasing breeds may be more likely to chase it and it only takes one snap to kill it. Never expect a cat to live with a dog that isn't used to cats (see how to choose a puppy). Terriers are fierce little dogs that may also have a go at a cat. Choose a gentle breed, like a Cavalier, so that the cat will be safe!
WHAT ABOUT VETS BILLS, AND OTHER COSTS?
Can you afford a large dog - they eat more and their vet bills are higher. Vets charge according to size.  All large breeds have a higher incidence of hip and joint problems. The giant breeds are worst for joint problems. They have a shorter life span - eight years is old for a great Dane.
Hereditary diseases in any breed make for big vet bills So check which breeds have hereditary diseases by reading Choosing the Right Dog for You by Gwen Bailey, if you can find a copy, or checking with www.dogbreedinfo.com
Miniature breeds may sometimes have problems like slipping knee caps. Dachshunds have back problems. White coated dogs are more likely to have skin allergies. Insure your dog for life with a good insurance like Pet Plan which covers all diseases. Cheaper insurance may not cover chronic conditions so check this out carefully. Breeders who say their breed has no problems are often ignorant. Don't buy a puppy from them. 
WHAT ABOUT HAIR AND TAILS?
Long haired dogs will require brushing every single day and will bring in more mud. You may need a car with air conditioning for very hairy dogs like Newfoundlands. They will suffer from the heat in a hot house so if you like high central heating, get a smooth coated breed. Poodles and bichon frisee will need clipping - either by you or by a dog groomer. All dogs will shed hair in the house so remember that black and dark hair is less visible!
It is now illegal to cut tails off dogs, unless the dog has a working dog certificate. So if a breeder is offering pet dogs with no tails, they are rogue breeders. Avoid them.