There are so many breeds in the world, and it's hard to choose the one for you. I personally want them all. But we all know that's nearly impossible. So you really need to break down what you want your dog to do. How do you want it to act? Do you want to wrestle or do you want a lap dog? Or if you intend on sporting, what breeds excel in that particular sport?
Decide whether this new dog is for you, your family, your job, a sport, or for a loving unbiased companion. What traits does your dog absolutely require? What can you be flexible about? What qualities are you unsure about? Do you want a big dog or a little one? Or somewhere in the middle? Do you want an affectionate dog? Do you want a smart dog? What do you want your dog to look like? What colour do you want your dog to be?
Now that you know what you want, what is realistic? What climate do you live in? How much can you afford to pay at the vet? How much exercise will the dog get? Will the housing be adequate for a busy border collie or the giant English Mastiff? How much money can you spend? How long can you keep your dog? I hate to bring this up but toy poodles live around 15 years on average whereas Great Dates only live to be around 7 years old. How much time and/or money can you spend on grooming? Will you spend a lot of time with your dog or do you work long days?
Now that you have a good idea of your perfect dog, take a few quizzes online. DogBreedInfo.com has a pretty good selection. And so does Eukanuba. If you just want a purebred though, animalplanet.com/dogs has a good test. Also watching shows like Dogs 101 and Breed All About It on Animal Planet are very useful.
Hope you find the perfect dog!