first of all I'd like you to read this http://islamqa.info/en/33656
Islam allows a man to marry Christian or Jewish women it doesn't matters you are young or not but the marriage has to be in accord with the Islamic tradition so you two can lawfully be husband & wife, I hope you won't mind it ma'am.
let's just say you are not allowed to marry him as a Christian and you have to revert to Islam only to marry him, only for him and this marriage and not for God and yourself, but where's the point in that ?
I think you two should go for it if you truly love each other and both are happy with this and you shouldn't worry about this right now. I know you have been a Christian all your life but yet I implore you to keep looking for answers and keep seeking guidance because God in Quran has promised guidance to those who really seek it from him with an open mind and heart. that's the point, it has to be your decision because this is your life and you are the one who is going to face the consequences of your choices & decisions and not anyone else hence I ask you to start learning about Islam with all your heart rather than just verbally converting to Islam so that you could married with the one you love, that's all I can say.
I hope this helped, I really wish you well and hope you find your way and everything works out perfectly for you two. I wish you both a very happy marriage and beautiful days ahead in your life, goodbye.