It depends on the kitten's age. But let's start with the basics. Get a warm blanket, bowls, toys, and food. The latter varies with the kitten's ability to eat. If it's already old enough to eat by itself, you can introduce it slowly to watered-down tinned food for kittens. Give it straight out of the can and it might give the animal stomach trouble. Kittens with diarrhea can die very quickly. Provide water and milk but introduce the latter in small quantities to make its digestive system used to the new diet. You can buy dry kibble later when the kitten's a bit older. If the kitten is too young to eat solids, you can purchase formula and equipment at your vet.
Watch out for fleas as they can make a kitten dangerously anemic, but also make sure that you buy a safe flea product should you notice your pet has parasites. Speaking of which, you need to deworm your kitten as well so that means buying a pill, paste or liquid that will provide a broad spectrum kill (kills many worm species at once).