The choices you make at the start are crucial. You have to ask yourself if you are prepared to devote the next ten to fifteen years of your life to the care of this living, adorable creature. Are you ready to give him unconventional love and attention, for that is what he will be giving you in return, if you treat him the right way.
If your answer is yes to these first basic questions, then you have to consider other choices, which are numerous. How big or small do you want him? Do you want an energetic or a docile pet? Do you like them long haired or short haired? Do you want him to be kid friendly? Do you want a pedigreed puppy, or a cross breed, or a saved one?
A lot of the choices will fall into place, if you consider where you live and how you live. For instance, is your home large enough for a Great Dane? Do you have a balcony or a fenced-in yard, or either? Are you an indoors person, or do you love going on long walks? Do your neighbors like dogs? Are there kids at home, or any other pet? Will there be someone at home at all times, or will your puppy have to get used to being home alone? Lastly, will you be able to afford feeding him, paying the vet's bills, and kennel charges when you go on vacations?
So do all the research you can beforehand. Talk to the local vet; if there is a local dog rescue center, you can tell them about your requirements; see the kind of dogs the neighbors have, they may have already done their homework, and their experience with their pet dog may give you valuable insights; read up on different breeds at the local library, the internet, or at the bookstore. Do not make the mistake of giving in to the kids' whims; you have to be totally comfortable with the choice you are going to make, since you will be carrying the brunt of nurturing the puppy!
When you are doing your research, note down any special equipment which may be required for the breed you will be choosing, like yard fencing, collars and leashes, bedding, food, toys etc. You have to know beforehand if your chosen breed is for any health problems which it might develop; you might even have to change your mind.
Lastly, never buy a puppy for someone as a surprise, and never buy one for someone without asking them these questions.