God knows best and I know very, very little but from what I've read, the scholars disagree on this issue. Every scholar says that intentionally abandoning the prayer is a kabair sin but there's disagreement on whether it makes you a kafir or not. So this is very very serious stuff. If I understand the issue correctly, the question isn't whether someone who doesn't pray will go into The Fire but whether they'll ever get out of The Fire (and Allah can forgive whoever He wills and punish whoever He wills).

But the good news is you're 13, which means you hit puberty only recently and puberty is when prayer becomes obligatory for us. So being frightened is a good thing, you should be (as should we all), but don't despair because you're absolutely young enough to turn this around.

I have no idea when I will start

You can start today if you want to. Do you know how to pray yet? If not, there are some good resources I can link, or you could ask your family members or imam to teach you. But whatever you do, don't tell yourself "I'll start tomorrow". We are very very weak creatures who have no knowledge of the future and even less control over it. You don't know that you'll have a tomorrow so if there are good deeds you need to do, start doing them right away.