Here's another option if you're on the fence:
Lots of shelters need foster parents for dogs who are adoptable but don't have a home yet. That way you have a dog for a month or two and have to take care of it, but don't have to commit fully (unless you love the dog and then just adopt it). We did this with our second one we adopted in August because we knew he had some issues but once he was in a good home and trained he is amazing.
And if you're going to get one, please don't do it from a breeder. There are TONS of good dogs who need homes at rescue shelters all over the place.
This is a very smart idea and will let you have a feel for the responsibilities.
If you really decide to buy a dog take some things into consideration:
Neighbors: if you're house is too close to theirs, if they are touchy or if they have a dog of their own than a small/yappy breed (which are MOST but not all toys) may not be a good idea
Weather: if you will have the dog mostly outdoors then you have to get a dog that can stand where you are, pugs, Neapolitan mastiffs, sharpeis, Chows and others do not thrive in the heat and chihuahuas, chinese cresteds and many small breeds do not do well in the cold.
Food: If you are only able to feed them once a day then you will want to stay away from large breeds that are prone to bloat/gastric torsion like great danes, mastiffs, Irish wolfhounds, etc
Energy level: if you wont have much time to walk them then you will want to stay away from active breeds such as border collies, boxers, husky, retrievers, etc you may want to look for a pug, a bulldog or so
Special requirements: some dogs need special care like bulldogs, mastiffs, pugs and french who require cleaning of the wrinkles, other dogs like most toys, chows, afghans, new foundland and others require grooming and combing.
Education: some breeds may be a little bit more difficult to handle if you do not spend time educating them and making sure they understand you are Alpha, certain breeds such as pits, amstaffs, husky, sharpei, akita, chows require discipline. Others are just not very bright and need additional training and more repetitions to understand commands afghans, bulldogs, pugs and others are not the sharpest tool in the shed
Puppy temperament: once you decide on a breed and you go see litters PLEASE do not go for the cute one, the one biting everything or the active one, instead try applying a temperament test to see which has the best qualities.
Children: This one is important since some breeds may harm children, not because they are aggressive but unintentionally, so if you have toddlers around constantly then dogs like danes and mastiffs may whip them in the face or get over excited and push them.
Useful links:
Animal Planet :: Guides :: Dog Breed Selector
INTELLIGENCE OF DOGS - The ranking by breed
Volhard Puppy Aptitude Test