Having your first period tells you that your body has changed, and you are becoming a woman who will be able to have a baby, if and when you want to. The period (the days that you lose blood through the vagina) is part of a 'cycle' of hormone and body changes.
During each cycle, the lining of the inside of your uterus (womb) gets thicker, so that if the egg (ovum), which is released from your ovary each cycle, is fertilised by a sperm, the uterus is ready to provide a place for the baby to grow.
A period is when the lining separates from the rest of the uterus because it is not needed for this egg to grow. The old lining is 'lost' and the uterus gets ready to make a new lining for the next egg.
The 'loss' is mostly blood, which can be bright red, dark red or dark brown, and sometimes has some clots (dark lumps of blood) in it.
How long a period lasts and the time between periods varies for different women.
The first period happens for many girls between the ages of 12 to 14 years, but quite often it is earlier (from as early as 9 years old) or later (up to 16 years).
Hope this helps.