I think it's 100% normal as a human being to fear death and to question our religion and how the world works because we don't know the answers to any of it. We have no way of finding out what happens after death, and no way to prove that God or any kind of person/figure from any religion ever even existed.
I'm 19 and I'm still trying to figure life out. And you said you're still young, so (I know easier said than done) try to stop stressing over all of this. You have plenty of time to think about death and all of that. Have fun with your life right now.
In my opinion, all of the theories and myths and stuff like Heaven and Hell is just a comfort to the living. We like solidarity, we like to know the answers to everything. So we comfort ourselves with the idea that some people to go to Heaven and some people go to Hell or the souls of our loved ones will forever watch over us, etc. etc. Funerals are partially for the living as well (also a way to pay respect to the one who has passed). But funerals proved a sense of closure, and they pull people together. They can cry and hug and comfort in the same place.
There are few people who aren't afraid of dying, and they're either lying or they're comfortable with the fact that it's a part of life. It's just how it works. Sometimes we get the chance to live to be 100 and sometimes we don't make it past being a baby. But when or how we die doesn't take away from how we should spend our life. We should still have the best life possible because that's what counts. Not our death. Everybody dies someday, it's inevitable. And honestly, I think repeating the truth to yourself is a start to getting over your fear. The rest comes with age in my opinion. Because as you age, you also experience. And you learn. Your knowledge and your opinion of the world change over time, and so will your fear of death.
As for why we're here.....we're here for a lot of reasons. We're here to grow, to learn, to make mistakes, to evolve, to develop, to create, the destroy, to love, to hate, to discover. There's no one direct answer of why we're here. We were given a chance to be alive and to do all of these amazing things.