At fifteen you have plenty of time; it's good to think about such things but you don't need to decide right this moment. As for a career, consider things that you like to do and discover careers and jobs that involve things you like. If you like music, consider that. You don't have to be a musician; just work in stores that sell music or instruments. Art; same thing -- you needn't be an artist but work with art. If you can have a job that at least partly involves some activity you like, you will have taken a step toward a happy life. In the mean time, don't be scared. You don't have to grow up all at once; just one year at a time like you always have.
There really is no rush. Some kids know what they want to be when they're 5 years old and that vision never changes. Me - I had no idea! What will help you here are 2 things. 1. Write down what interest you - what you like / love to do. 2. You can get the book Strength Finder (I found one at Target). You need a new book - not a used one. Read the book (it's really quick) and open the code in the back. It will direct you to their website. Spend a few minutes answering questions and it will help you narrow your focus. Good luck!