At your age, it's becoming more and more important for you to establish an identity for yourself within your family, outside of your family, and with your peers. You can certainly tell people, "My mom is Christian and my stepfather is Jewish," but I think you're concerned with what to say when they respond, "Yeah, I get that, but what are you?"
You know that, traditionally, a person isn't allowed to be both a practicing Christian and a practicing Jew. So you might be somewhat confused and scared about what you really are in terms of being a member of a religion. You don't seem to have been given enough guidance, direction, and support to feel comfortable and secure in this important area. Maybe your mom and stepfather don't know how much this bothers you. They need to know your concerns. Don't be afraid to discuss this topic with them -- I'm sure that they don't want you to continue suffering because of it.
My guess is that you need to feel that you "are" something concrete when it comes to religion. You may, deep down, even want and need to feel a part of one religion and embrace it rather than not really knowing what you are. I know that you might be afraid of disappointing either your mom or your stepdad by wanting to be considered a member of a religion that is not theirs. You should not have to live with this or any other fear related to this issue.
So, please talk to your parents about your needing real answers to your questions about religious identity. And if you want to pursue a particular religion, let them know. It's their job as loving parents to help you with this very important part of who you are. If you need more help with this, please write me.