It isn’t easy being 15. I remember it well. The fact that you are asking this question may point to your purpose as a thinker, a philosopher. It shows that there is something stirring in you that can make life very interesting and exciting. A soul-filled life brings its own reward. But, as Socrates said, “The unexamined life is not worth living.”
Start reading about spiritual subjects and see if any of it appeals to you. No knowing what you want to do can be a place of creativity and growth. A great place to start would be some of Wayne Dyer’s books. His book, I Can See Clearly Now tells about how he found his purpose as a spiritual teacher in this life. He went through a lot of struggles and he will be able to guide you.
Here is an introduction. From there, you might want to get the book, listen to the full audio, or find it in the library:
Wayne Dyer - I Can See Clearly Now (Chapter One)