My girlfriend and I are very careful when doing sex. Apparently, not careful enough. She got pregnant. I used a condom and she said she was on a pill, what happened? And what do I do now?

I really like this guy and now he has asked me to have sex I am under age and I don't really want to do it but I really like him but he just wants to mess about and I want more of a committment. I really don't know what to do, so confused. HELP PLEASE.