New cat owners have a long list of items that they must buy. Near the top of the list of things you will need to do will be buying a litter box. If you plan to bring a cat into your home, this should happen BEFORE you bring the cat home. Most of the time, cats are going to start exploring their new space.
One thing that they will be searching for is the most suitable bathroom. If the litter box is not yet in the house, they may choose a plant or some other space instead. It is easier to teach a cat to use the litter box than it is to teach them to STOP using some other spot.