Female cats can become pregnant anytime after about 6 months of age. However most females aren't psychologically mature enough to become a mother until they are at least 1 year of age.
A female, once sexually mature, will go into heat (also known as estrus). Cats are seasonally polyestrous which means they will go into heat multiple times a season until they mate. Season usually starts a few months after winter solstice and usually ends in September or October. Indoor cats who are only exposed to artificial lighting may stay in season year round! Estrus itself will last 4 - 10 days and will cycle (about every 2-3 weeks) until the female mates, is spayed, or goes out of season. During this time the female will become fairly agitated: thrashing about, rubbing on floors and furniture, spraying, rolling about restlessly, and crying or meowing loudly.
If the female does mate and become pregnant you can expect her to give birth in about 62-69 days. Three weeks into her pregnancy you will notice her nipples becoming pink, firm and erect. By the end of the fourth week you will notice her belly enlarging. And anytime after the seventh week you will be able to feel her kittens moving about inside her belly.
Pregnant cats do need a bit of special care, although not much. Make sure to keep her indoors and in a clean, safe environment at all times. If she is an outdoor cat and must be outdoors it is imparative that she at least remain indoors for the last 2 weeks of her pregnancy. This is for her protection as well as her kitten's protection. Also switch her food to a special diet for pregnant cats. Consult your vet for what brands and types of foods will be best for her. Make sure her litterbox is always clean to avoid the spread of infection; also you may want to switch to a 'shorter' litterbox as her belly starts to grow. This will help her get into and out of the box. And lastly you will want to prepare a 'maternity bed' for her.
The maternity bed is meant to be where she will give birth. If she doesn't have a maternity bed she will find another suitable place which could be your closet. So it is best to try to get her accustomed and comfortable with this special bed. You can create a maternity bed out of a cardboard box which is at least 2 X 3 feet wide and about 1 1/2 feet high. Also you will want the box to have a top to it (this will help keep warmth in for the kittens and keep it dark and quiet). Cut a hole in one side of it that is large enough for mom to crawl in and out of. Place a few inches of shredded newspaper in the bottom of the box and then cover that with a blanket or old shirt that your cat is comfortable with. Make sure to place the box in a warm and quiet place.
Once your cat is close to her due date make sure to watch her for blood spotting or any signs that could indicate a problem pregnancy or a premature birth. Also watch for if she is 2 or more days overdue for her delivery, this can indicate a problem pregnancy. If you at any point during the delivery think that something is wrong such as if she is straining too hard without delivering any kittens, bleeding, or producing a discharge that is foul-smelling or discolored don't hesitate to call your vet immediately. During her last week of pregnancy make sure she is kept away from other pets, children and stays fairly solitary. This is for her cleanliness and safety (females in late stages of pregnancy can get aggressive). Try to keep her calm and as inactive as possible during this time period and try to get her accustomed to her maternity bed.