The main drift is that two weeks before the next period, you pop out an egg from one of your ovaries. It is viable (able to get impregnated) for about a day, and it then washes out if there is no sperm waiting for it, and in two weeks a period comes. Many women have a period about every 28 days, which puts ovulation at day 14 or so. Many other women have longer or shorter cycles, and some have irregular cycles. But the ovulation leads the next period (rather than following the last), so if a woman has, say, a 35-day cycle, she would ovulate around day 21.
It's a good idea to keep a record of your cycles, when they start and end (I used to use graph paper) and note any cramping or twinges at mid-cycle. You'll be able to tell your doctor "my cycles are 29 days long" or whatever, and should be able to tell from your changes in cervical mucus, roughly when you ovulate.
One thing that is also a good idea for you to do is to look up sites designed specifically for young teenagers that are educational on the topic of sex education. (I mention "that are educational" because if you just google sex information, you'll get a lot of sleazy porno sites coming back. Try "sex education for teenagers" or like that.) It is really useful before you are too much older to understand some of the myths, (untrue statements likes "a girl can't get pregnant if she has sex standing up" or "you can't get an STD from oral sex" and a whole bunch of other dumb stuff that kids believe). Knowledge is truly power here, and if your boyfriend someday pulls some of this stupid stuff on you, you'll be able to keep from getting pregnant because of what you know.
I'm not saying I think you're being asked for sex at your age and hope you don't get put into that position for a long time, but boys' hormones push them and they push their girlfriends, and in our highly sexualized society they tend to ask for it younger than ever because they see the implication in the media that it is OK. Your mom will help you and your dad, but what will protect you the most from being taken advantage of is your own knowledge.
To add to what is above... the first day of your cycle starts on the first day of your period and ends the day before your next period starts. Sperm can live inside you for 4-6 days after you have sex. Therefore you can have sex 6 days before you ovulate and still get pregnant.
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