Though the window of time between your first ovulation and your first period is very small, it is still possible to get pregnant during that time. Even if you have never had your period, you should still be on birth control if you are sexually active- also always remember to use a condom, birth control doesn't protect from STD's!
It's possible if you're ovulating. You ovulate before you have your period so if you were to have unprotected sex during ovulation before your first period, it's possible.
Theoretically yes, because you have to have an ovulation before you have your first ever period. The chances for you to hit the exact time of ovulation for your intercourse are lower than with a regular cycle. To prevent pregnancy you should always use a condom, even if you haven't had your first period yet.