In the first weeks of life, kittens are entirely dependent on their mother's milk and do not require any additional feeding.
Occasionally, the mother's milk supply is inadequate. In such a case, feed your kitten a cat milk substitute available from your supermarket, pet store or from your vet. Don't use cow's or goat's milk. Both have very different nutrient profiles than cat's milk, and should not be fed to a kitten.