A female cat will have her first 'season' at about 6 months of age, and at this time she becomes sexually active and ready for mating. She will start to 'call' for about 4-7 days in every 21, and during this time she will become vocal and may show changes in temperament and posture. Cats tend to come into 'season' at particular times of the year.
The only way to ensure that unwanted pregnancies do not occur is to have your cat spayed at about 5 to 6 months of ago. Every year, countless unwanted kittens and cats are destroyed, so think carefully about homing any kittens which are produced before your decide to breed from your cat.
If however you do wish to breed from your cat at some future date, then a course of medication to postpone 'calling' may be recommended and prescribed by your vet.
Likewise, a tom cat will become sexually mature at about 5 to 6 mnths of ago, and at this time he may start to display 'male' behaviour such as roaming, fighting and spraying. Such behaviour may cause problems for the owner and cat, and can be largely prevented by having your tom cat neutered.
Your vet and practice nurse will be happy to advise on neutering or spaying your cat.