How do I know what to do when a cat just has kittens?

Keep the kittens and the mama safe and warm. The mama should allow the kittens to suckle her milk. If the mama is especially exhausted you need to make sure all the kittens find their own teat and that none of them wander away. If you have any problems contact/bring them to a vet. If they are closed contact some of your friends or if you are a kid your parents and they might be able to help you. Good Luck!:)