It's normal to be feeling like you're unsure of things at your age or at any age. Death is a scary thing because of the unknown. Even people who believe in Heaven can still be scared. Faith doesn't mean that you can't question things. Never let anyone tell you that you can't ask questions and be uncertain about things. It's normal. God gave us reason and an inquisitive mind. Why wouldn't he want us to use it?
It really helped me to study philosophy to really work out for myself what I believed, rather than taking it from other people. Are you in school? If so, I would recommend taking a philosophy class or just doing some reading on subjects that you are unsure about.
You can even go to a local church (or synagogue or mosque or whatever) and talk to the minister (or Rabi...). Find one that is willing to let you ask questions and have a discussion with you (rather than just quoting scripture or giving you a pat answer).
Also, try reading the Bible as literature, rather than as literal. It really opens up the different meanings and possibilities of the words. You have to decide what things mean for yourself. Open your mind to all the possibilities.
Worrying about these things is normal and you can't just stop because someone tells you to. Having faith may make the afterlife less scary, but losing family is never going to be easy, no matter how sure you are that you will see them again. All you can do is make sure that your family knows that you love them and to spend time with them. If something happens, yes you will miss them terribly, but you are stronger than you think. You need to have faith in yourself, as well.
If you have any questions or anything, feel free to write back either on the forum or in a personal message. I hope some of this helps. Just try not to get overwhelmed. Take it one step at a time.