Certain things like food, shelter (i.e. an aquarium or cage; have fun figuring out where to place your fish if you forget the aquarium), a leash and collar (for a dog) and cat litter (for your feline friend) must be purchased right away. 4- Take the pet to the vet If you are buying a pet such as a dog, cat or bird, I recommend you take it to the vet as soon as you purchase it.