The most important thing in general is that you should really do your research before getting a dog, period. People should research canine training, behavior, nutrition, proper care, health, the right breed for them, where to get a dog for and what places to avoid buy from, etc. There's just so much that a dog owner should know before getting a dog, and too many learn this the hard way. There is also so much conflicting information out there, and I think it's best if people figure out for themselves what to believe before getting a dog.
If I had to pick just one thing however, let me say that one of the main things I wish that people knew before getting a dog is what to look for in a breeder.