Always see Mum!
Except in exceptional circumstances, you should always be able to see the mother of the puppy. Visit where the puppy was born, regardless of how far you have to travel. You should aim to visit the puppies at least twice; usually when they are 5-6 weeks old and then when you collect your puppy at 8 weeks.
A responsible breeder will let you see all the puppies in the litter, with their mother and you should be able to handle them. You may not always be able to choose the puppy you want, as the breeder will have probably picked their first choice to keep and other buyers may already have chosen, before you.
Never buy from a pet shop; they usually get their puppies from puppy farms. Get a written receipt and registration paperwork for the puppy. Carefully read and understand any conditions imposed by the breeder.