We don't know when you are fertile. Even women who have regular periods often don't know when they are fertile. It's easier to guess when you're fertile if you have a regular period, but sometimes it's tough to know for sure if you're right.
In a regular cycle (28 days), ovulation (when you are fertile) occurs right in the middle, around days 14-16. So, it's kind of right in the middle of the last period and the next one. If you have an irregular period, it's that much harder to track your fertility. They make things like fertility monitors for that. One thing that you can look for is stringy discharge that looks kind of like egg whites. That is the easiest way to know that you are probably fertile. If you do not want to get pregnant, you need to always assume that you are fertile and use birth control every time.
Birth control reduces your chances of getting pregnant. It does not eliminate them. So even if you use the birth control exactly as intended, there is still a small chance that you will get pregnant. To reduce your chances even more, you can use two methods of birth control. If you are going on the pill, you need to use condoms for at least one week while the pill takes effect. You can get them at the store or your ob-gyn might give you some. You can also get spermicide over the counter. Spermicide is not great on its own, but it makes a good back up method.
