There are many dog breeds to choose from and also cross-breeds and mongrels.
The advantage of a choosing a pure bred dog is that they have fairly fixed characteristics and temperaments and so it is more easy to find one that is almost certainly right for a particular environment, lifestyle and requirements. Choosing to buy a cross breed or mongrel means that the characteristics and temperament are more uncertain and if buying a puppy, it may not be possible to ascertain the adult size of the dog although if its parentage is known an educated guess can be made.
It is important that thought is first given to the type or breed of dog suited to the potential owner's circumstances. A big dog means big expense, whilst a smaller dog is cheaper to care for on a daily basis. A large dog requires space, and may also not be so suited to households with smaller children who may get knocked over by a large dog. Some dogs require more exercise than others, some can need firm handling and are not so easily trained as others and so are not an ideal first dog. Some breeds crave company and can be destructive is left alone, whilst others are more independent and will happily amuse themselves if left on their own for short periods. It is important to thoroughly research the various breeds of dog to ensure that whichever dog chosen will fit happily into the intended lifestyle, environment and suit requirements and to be aware of any genetic defects to which the dog breed may be prone.
The age of the dog is another important factor to consider when considering obtaining a dog. For many the idea of obtaining a puppy is appealing but a puppy will require someone at home all day, or periodically throughout the day, as it will require several feeds and will also need to relieve itself frequently throughout the day and night. A puppy will also require training and is likely to chew items as its teeth develop. For some, therefore, a boisterous, untrained puppy can be a strain and so a house trained young or older adult dog is more appealing.
The sex of the dog should also be considered. Generally females (bitches) demand more attention, are easier to train but come into season twice a year when they can be messy and try to escape to receive the attention of male dogs. Neutering is a solution but can be costly and such costs will not be covered by veterinary insurance. Males (dogs) tend to have more spirit and so can be more difficult to train but are generally more consistent in temperament. They may also be prone to wandering when bitches are in season within the area.
Once the breed or type of dog, age and sex has been decided, thought can be given to obtaining the dog from a breeder or other source.
Puppies should not be sold or permanently separated from their mother until 8 weeks of age minimum. When buying a puppy look at all puppies in the litter to ensure they all look healthy and active. If one puppy is sickly it may indicate some genetic weakness or contagious illness which may be present in the others and so these are best avoided.
Any puppy should be curious, alert, bright eyed with a clean coat. It should be inquisitive, not too nervous and not show any aggression. Beware of any puppy that has runny or sticky eyes, runny nose, sneezing, wet or dirty bottom, matted fur, seems lethargic or does not have a firm body. All of these things can indicate a sick puppy.
Watching the puppies for a while will show their individual personalities as it will undoubtedly become apparent that some are more playful, confident, boisterous, quiet, wary, etc than others. Handling and playing with the puppies will also give the opportunity to see how they interact with humans.
If when viewing puppies you are concerned about their environment, or the welfare of the mother or puppies being offered for sale then you should inform an animal welfare organisation.
If looking at an older dog, perhaps a rescue dog, the same applies - look for one which is curious, lively, friendly with no signs of aggression, with good clean fur and bright eyes.