Before you bring home a new dog, it would be prudent to decide who will feed, water, and walk the dog, and where the dog will sleep. Who will keep track of the dog's care, shots, and medications? Who will do the house-training (good times)? What other rules should you update family members on?
The Humane Society is an excellent place to pick up a "rescue." For a flat fee (generally a pretty reasonable fee at that), you can secure a dog that has been spayed or neutered and had its first round of injections. You will also have the satisfaction of owning a dog that has been rescued.
The City Pound is another good place to go. The negative aspect of this option is that the dog probably still needs to have all of its shots, to be fixed, and to be checked over by a veterinarian. The positive aspect is that you will have saved an animal from an almost certain death by injection.
Generally you are at somewhat of an advantage when buying an animal through a classified advertisement, as the seller will usually know the breed, age, and disposition of the animal.
If I were buying a dog from advertisements, I would buy an older dog because those seem harder to place. Then again, it would be difficult or impossible to buy an older dog and not wonder about its mortality. I guess that just depends on your level of sensitivity and what you're looking for in an animal.
When considering breed, consider any experience you've had with particular breeds, and any wisdom you can glean from other experienced dog owners. You can also research breeds of interest on the Internet, and talk to reputable dog breeders.
Consider how many children are in the house, and how many children are frequent visitors. Would they try to saddle up a Great Dane, a Rottweiler, or even your poodle? Would they lounge and drool on or attempt to shave Snoopy the weenie dog?