To keep your puppy happy, you will need to buy supplies from the various shops in Nintendogs. They sell food and water, as well as toys and accessories for your puppy.
One of the shops that you are likely to frequent a lot is BARC Supplies. You can get there by tapping Go Out on the bottom right, then tap Shopping, and tap BARC at the top left.
The supplies here include food, water, treats, and toys.
Mr. R's Second-hand Shop
When you go out on walks, you might find recyclable materials lying around. You can take these to Mr. R and have them exchanged for other items, like a leash, toy helicopter, keyboard, and more.
The kennel is where you can go to buy new pets. You can raise money for new pets by doing competitions.
This is where you can buy things like collars and bows for your puppy.