Are you considering buying a dog? First, please, please think about adopting one from a rescue group or a shelter. There are SO MANY wonderful dogs just sitting there waiting for someone to take them home. It's completely untrue that dogs in shelters end up there because they have behavior problems. Some of the reasons are: divorce, financial problems, moving to a place that won't allow dogs, children are allergic, and no time for the pet.
If you are certain that you want a puppy, here are some tips to help you to screen out the good breeders from the not-so-reputable ones. Best of luck with your new friend!