For me, the best place to start is a serious think about:
a) how much walking you can and WANT to do every day
Expect even the laziest of dogs to require about an hour a day, best split into 2 x 30mins. The most to require a couple of hours and some of the most active breeds to require even more. This is worth serious consideration because it has to happen 99% of the time. That could mean cold, dark, rainy walks. Windy walks. Muddy walks. Walks when you feel sick. Walks when you have a really busy day ahead and so have to get up at 5am to fit the dog in then walk him again at 10pm when you just want to go to bed. Walks when the kids are playing up. Walks when they are sick. You get the picture 
b) how much time you can and WANT to dedicate to a dog every day
This includes grooming, training, playing etc. Again, some dogs might be happy to curl up and snooze most of the day away but others (active, young) will find thier own entertainment if you just expect to walk and feed them but do little else.
c) which of the typical dog behavious you definately could not live with.
Barking, digging, chewing, brining mud into the house, taking a while to toilet train, needing to be around you most the day, hating to be left alone, not good with cats or other small furries, poor recall, pulling on the lead. All of these things can be imprived by training but that takes time and is harder with those dogs prone to do these things - e.g. teaching a Jack Russell not to bark at the door is much harder than teaching a naturally quieter dog.
d) looking at the last year of your life and counting up how many No Dog days out you've had (anything 4+ hours) and thinking about what you would have done with a dog for them. Perhaps you'd be happy to give up all those trips to a musuem or maybe you have friends who are happy to dog sit for foreigh holidays. Or maybe not and having the dog will feel like a real tie. This could be especially true if you get a dog that doesn't like being left alone at all.
Thinking through those things will give you an idea of whether or not getting a dog is a good idea and also start to guide you to what type (age, breed) of dog.