Okay firstly, you are way too young to be thinking about marriage. Both of you. I don't care how mature you are, you need to think this through properly. Think about whether you both will be compatible in the long run, whether you think she will make a good mother to your children, whether she is responsible and mature to deal with trials and tribulations you guys may face in the future. Marriage itself is a huge deal, muslim or not and to take it so lightly does it injustice.
Secondly, what religion is she? From my understanding muslim men can marry christian or jewish women, I'm not sure about this please speak to a scholar/imam. It seems, as young as she is and after finding out that you can't marry a non-muslim, she might be wanting to convert for you. Which begs the question, does she fully understand islam for the religion and way of life that it is, or does she just want to accept what she needs to accept for you? Because the latter can be detrimental and cause a rift in your relationship. Would you really want her to convert to islam just for you rather than out of love for God and his Prophets (peace be upon them)?
You need to take some time and think about this, there is no rush to make a decision just yet. I'd strongly recommend talking to your parents about it too, if that is not possible as you're afraid of their reaction then it speaks volumes about the fact that you're unfit for marriage.