You think it over with yourself or with your partners or room mates, what would be the most fitting pet according to lifestyle, and then you choose.
For example a dog requires at least three walks a day. Is there someone in the house to take the dog out? If you have a very small appartment maybe you should choose a smaller dog. If you have a huge yard maybe you should consider adopting something a little bigger.
Are there children? You want a dog that will be children friendly.
Do you want a dog that is already house-trained, or are you willing to take a very young puppy and train it?
And we haven't even started talking about cats, let alone less popular pets like rabbits or a pony...