If the puppy is not home yet, or arrived in the last few weeks, start with Getting a Puppy?. Roughly half of it covers decisions you make before the puppy arrives, so it is worth reading early.
If your dog is past that stage, whether that is eight months or nine years, start with How to Become Your Dog’s Best Friend. It assumes the dog is already home and picks up where the puppy guide stops. Read together, they cover the whole of ownership.