The most important question you can ask is this: are you willing to commit to a multi-year relationship? One that could last up to several decades? Dog ownership is a serious responsibility.
Don't forget that you may be all that stands between your pet and the possibility of being a pound dog.
So think hard.
This is not some tiny little fling where you can just "give up" the dog when you are tired of it.
This is for the dog's entire life, as well as a significant part of yours.
Make certain that you realize what you are getting into.
What do you see as your dog's purpose? Do you want a lap dog or a guard dog, or just a companion? Are you willing to train the dog or have someone else do it if you don't have the time? The size of your dog may be an indicator of how long it will live.
As a rule of thumb, the larger the dog, the shorter its life.
Though there are always exceptions, most dogs live from one to ten years though some have even reached twenty.
The next most important question you can ask is this: what kind of dog would be best for me given my life style? If you are a workaholic who is never home for much beyond sleeping and eating, a dog is probably not for you.
Take into account how many hours a day the dog will be alone.
You will do best if you get a dog based on the way you live rather than on how you would like to live.
Every dog is a major time investment.
The way you live includes where you live.
Do you own or have access to a place where your dog can exercise and go potty? If you own a home with a yard, you will have instant potty facilities.
If you rent an apartment, you will have to think about how far potty facilities are from your door.
While no one loves barking dogs, apartment neighbors may really dislike your dog (and you) if it barks while you are not there.
Talk with other dog owners who have dogs you like.
Can they give you tips on how easy/hard their dog is to live with? Is it happy with adults and children? Does it require enormous amounts of food? Does it need lots of exercise? Does it sleep a lot? Are there lots of vet bills? Do you need a dog license? What are the costs of keeping the dog? If you are a cleanliness nut, you will probably not want a long-haired dog which sheds.
If you are allergic to dogs you will be severely limited in the dogs that you can live with, but you can look for a dog with a single coat or hairless dog.
If you own rare breakable items, you will want to be assured that your pick will not destroy your collection.
The size of your place could be the deciding factor for the size of your dog.
While many very large dogs do well in small spaces, you may not wish to share so much of your people space with your dog.
Some things that most people look for in dogs include handsomeness/beauty of the animal, though many will tell you there is no such thing as an "ugly" dog.
Is this a dog that you intend to keep washed, brushed and well-groomed? Then whether it has long or short hair will matter.
Have you ever bathed a dog? Are you willing to learn how? Do you travel enough that you will need to find someone to "dog-sit" for you while you are away? Or will you board your dog at a kennel? Do you have friends that you could swap "dog-sitting" with? Many other questions like this will no doubt come up when you talk with someone who has a dog that you like.
You can explore the American Kennel Club's site to find general information about dogs and the AKC links to the websites for specific breeds.
You will also find information about available puppies and possibilities for rescue.
Responsible breeders will be willing to answer any questions you may have about their dogs.
Good luck on finding a good fit!