Well first off you are with out a doubt too young to be worrying about this yet, as am I and unless you have a terminal illness I really don't think many people on this forum really need to be worrying about the life ending quite yet, you have many, many days ahead of you. Still like you I too am interested in the thought of it, but less so worried about it as such. Personally I am without religion and as such I see the afterlife as unrealistic. There is no soul, scientifically your conscience being is simply an illusion of different senses and different 'chemicals' working away in your brain. Once we die our brain stops working and this conscience illusion falls apart.
However its still nice to believe that something MUST happen, even if it seems illogical, to me though it is not heaven. Heaven makes so little sense to me and it is illogical in so many ways. I prefer the belief in reincarnation, it's slightly less ridiculous. Not to mention that the elements of your brain make up who you are, so in theory if some of those elements eventually became apart of a different brain would that conscience belong to you? Or would it just be a new illusion that isn't really controlled by anyone? Who knows.
On the other hand if there is no form of afterlife just hope you live to an old age and have fun I guess. If you are old your standard of life will be much worse then it used to be and so as long as you have no (or little) regrets then death can be seen as a chance of relief, a period of rest where you no longer have to worry about the struggles of life. Or perhaps it will be a real bitch and you will wish you could go back and do it all again, if so I hope that there is some afterlife for you. To be fair it's just something that will happen and you shouldn't try to get worked up about the inevitable.
But once again, you shouldn't get worked up about it because you are still so young and death is still so far away. On the note of missing friends and family, be grateful for what you still have and that you are still in good company of the living. Best not to make good company of the dead, if you know what I mean.