I agree with all that has been said. Most importantly is to keep her STRICTLY indoors until the babies are 8-12 weeks and she is spayed. Intact males can get her carrying multiple litters at once and she can become prego again a week or two after giving birth which is extremely hard on the mother.
Please consider having her spayed right away unless she is too far along in her pregnancy. If she cannot be spayed right now, start gathering kitten supplies, feed her high quality kitten food only, save up a bit of money to have the kittens vaccinated (i think there are 2 or 3 sets of shots they will need) they will all need to be spayed and the mother too, dewormed, possibly defleaed if the mother goes outside.
there are a lot of things to think about when your cat gets pregnant, lots to prepare, lots to save for, etc. Skim through the pages here on what to do if the mother rejects the kittens and you will need to bottle feed them every 2-4 hours for them to stay alive.