Cats typically go into heat for the first time
around 6 months of age, but don' think you can safely leave her
surgery until that time, in can occur a month or two earlier. A cat
this young is too young to have a litter and probably is too young
to be an effective mother. Most vets will wait until 6 months of age
for an indoor cat, but if she is going outdoors all night long, many
vets will spay a female as young as 4 months old.
They go into heat, but they don't bleed the way a dog does. Most
cats I know have gone into heat before they were 6 months old.
If she's older than 3-4 months old, you can either go ahead and
get her spayed, or you need to keep her inside 100% of the time.
Cats are known as "induced ovulators", meaning that the physical
act of mating is what causes their ovaries to release eggs. As
soon as they go into heat, they are ready to mate, which means
by the time you realize your cat is in heat it may be too late.
Help control pet population, please have your cat spayed or
neutered.