Male
Once a male kitten becomes sexually aware, which can be as young as six months old, he may start pungent marking of his territory.  Cat urine has a very strong smell and is corrosive and unless you want to breed from your cat, it would be best to have him neutered.  This is quite a minor operation if done early - usually at six months.  Neutered males are very clean and home-loving. 

An un-neutered male will wander in search of females, get involved in fights and will often return battle-scarred, requiring veterinary treatment for abscesses and other ailments.    he is also more prone to catch Feline Immuno-deficiency virus - a deadly viral disease which is spread by bite wounds.  Neutering your male cat also stops testicular tumours occurring.

Females
Female kittens can come into season from as early as six months.  Cats usually come into season (ie are reproductively active) between March and September, though this will vary a little depending on weather conditions, such as day length and day light.  However, with the majority of cats kept indoors and not truly subject to outside weather conditions, some cats will even come into season in the autumn and winter periods.

During the breeding season a female cat,  which has not been spayed (a Queen) will come onto heat every three weeks.  When on heat, a female cat is usually extremely vocal and it is not unusual for an inexperienced owner to think their cat is crying in pain.  She is usually very affectionate, even more so that normal, and will rub against your legs and furniture.  She may also adopt a characteristic position, called "lordosis".  In this stance, she crouches down on her front legs and holds her rear end up, in a position ready to mate. 

If you do not want your cat to have kittens, it is advisable to have her spayed.