You ovalate once a month. Approx 7-10 days after your period you can get pregnant for about a week. That's the only time you can get pregnant, during ovulation. If you've been doing this outside that window, that would be why. Go live your life, have fun and date many guys and you'll eventually find "the one". You're so young and you have so much to experience before you have children. It took me many many years before I found my "one"