I didn't know what I wanted to do when I finished school.
When I finished school, I still didn't know what I wanted to do. I went to college to do Computer Programming, but I quit because it wasn't what I wanted to do, it was my dad's suggestion.
I went back to studies in 2013 and I'm just about to start my final year of my BSc Psychology Degree, but after that, I don't know what to do.
I know I want to write. I know I like learning. I have many interests. Psychology (of course), photography, ancient history, classics, languages, and more.
There are people out there in the world that are dead-set on becoming a doctor, or a lawyer, or a dentist, or an astronaught, but there are people that don't know.
But, there is nothing wrong with it! You don't have to have a career, so long as your happy and healthy, that's all that matters!
Ideally, get a job that you love doing! So if you love coding/programming, maybe look at doing something with that?
Or if you love law, look into that?
Or if you're not sure which one to choose (if you must choose only one), either do some work experience in these areas to get an idea of what you'll be doing or write a pros/cons list.
But you're 15, you don't have to have everything worked out at that age.
I'm 29 this year, I still haven't got it worked out. ;)
Good luck!