Menstruating or having your period means that blood comes out of your vagina for a few days.
Girls will start to menstruate (the menarche) between the ages of 9 and 16.
You can use a sanitary napkin or a tampon to collect the monthly flow.
It can take 3 to 7 days, most girls bleeding for about 4 or 5 days. You normally need about 3 to 5 tampons or sanitary napkins a day.
Once you start menstruating and don’t want a child (yet) you must use birth control every time you have sex. If you do it once without some method of birth control you might become pregnant.

Where does the blood come from when I have my period?
How long does a period last and how much time is there in between periods?
I loose a lot of blood when I have my period. What should I do?
Now that I have been on birth control pills for three years I start skipping periods. What’s wrong?
Can we have sex when my girlfriend has her period?

See also: premenstrual syndrome and menstrual cycle