Do you remember when she started crying and wanting a lot of attention? Putting her tail up and to the side? If so, the birth will be approximately nine weeks from that time. There is an almost foolproof way. Get her average temperature with a rectal thermometer. (Use vaseline, and hold her perfectly still if you are going to do this. Have someone else with you!) It will be around 101. When you think the nine weeks are almost up, check her temperature often. When it drops a degree, she'll probably give birth within 24 hours.
You should have a warm, private, dark place ready for her. I pad a box with towels, and put a layer of newspaper on top. If she approves of the spot, she'll start tearing up the paper and lying in the box. She will probably check out other spots in the house too. Don't palpate the kittens. Just rest your hand gently on her tummy, and if they move, they move. When she is done giving birth, change the towels and the padding also, if necessary. The kittens move visibly by 7 weeks.
The last couple of days she will want lots of attention, if she's like my cats. I check her private area. She will have a spot of blood often during the last couple of days, and then a bit of mucous. Eventually there will be more mucous. Finally, she will begin to have contractions. Sometimes a new mother, especially a very young one, will have to be taken to the box. Have the vet's number handy, and it's wise to know what to do if mother does not take the sack off of the kitten. The kitten has to have the sack removed in order to breathe. I keep clean towels ready to rub up the kitten if Mother cat has her kittens fast. There are many other things you can do, but I would get a book now and start reading. Usually the mother will do everything on her own, but it pays to be ready for emergencies. Keep the emergency 24 hour number handy!
We strongly recommend spaying and neutering pet cats. There are millions put to sleep every year. Good luck! Let us know if you need more help, but I know you realize we can't take the place of a veterinarian!