GREAT QUESTION!!!
I much prefer getting a dog-pound dog, than buying one. I also prefer choosing ADULT dogs over puppies, as puppies are so so so much more work, in my opinion.
but, you are so so smart, to want to know more about what breed to get.
I think the main thing to focus on, is the energy level of the dog. Some breeds of dogs need lots of exercise or the dog becomes destructive, and chews up your house.
some breeds are happy enough with only 2 walks a day, and some play time.
HOW ACTIVE ARE YOU? Are you a runner, or a jogger, or skate boarder or a bicycle rider?? Or, do you spend most of your time reading, studying, watching tv, etc etc.
THIS MATTERS a LOT in what type of dog you might be happiest with.
If you spend most of your time playing computer games, and bring home a "high energy" dog, you AND the dog will both be unhappy.
If you are a runner, and want a dog to run with you, but you bring home a "low energy" breed of dog, then you might be disappointed that the dog can not keep up with you.
HOW MUCH TIME will you have to exercise the dog and train the dog? How many hours a day will your dog have to be home alone?