The number of types of pet supplies is as unlimited as the different types of pets you can have. You should do research on what you need and why. Then talk to an associate at a pet store. If you already know why you need certain things, you can then make sure you are not talked into buying something you do not need.
One thing to keep in mind is that any good pet store that is truly pet friendly will give you a discount on your supplies if your pet is a rescue. They do this to decrease the number of animals that need to be put down at a shelter. Some stores have been known to give certain items for free or give up to eighty percent off all of your costs.
Many people are tempted to go crazy with new pets, just like with a new baby. They buy way too many toys, most of which the animal will never play with. Dogs and cats are surprisingly fond of very inexpensive things. You could buy a beautiful pet bed and the animal may never use it, instead preferring a cardboard box or just a corner of your home.
This can be frustrating, so just pace yourself in the beginning. Buy only what is needed to make your pet comfortable and safe. If it means a great filtration system for fish or a strong leash for a dog or a scratching post for cats, you should spare no expense. Animals can be entertained easily and amuse themselves. Your money is far better spent ensuring their safety.
You should also buy things that can help you train your animal. You need good litter boxes or a kennel--and most certainly you need baby gates to keep the animal from going in rooms you don't want. Training needs to be done early, so buy these things before you even get the animal to ensure there are no complications.
When it comes to food, ask what the pet has been fed in the past weeks by whoever is currently caring for it. Animal digestive tracts are extremely sensitive, so the animal could get sick in a number of unpleasant ways if they are fed different food.
If you wish to wean them off the old food, buy a bag of it and slowly start incorporating the new food. A small percentage increase every day will allow them to get used to it over two weeks to a month. Many people do this anyway in order for their animal to get the benefits of more expensive food, while at the same time making it last longer.
Never feed them human food. It is bad for them. Stick to water, too. Do not give cats milk, despite the rumor that they like it. Just because an animal CAN eat human food, and may even beg for it, does not mean that they understand their health. Chocolate is especially dangerous for animals, but this will not stop them from wanting whatever it is you are eating.