Wow, someone's a bright 15 year old! (When I was 15, I spent the entire summer playing video games and chilling with friends... -.-)

You can approach it from many different ways, but I think a good way is as Timgt5 says: look at all of your interests and ask yourself which ones you enjoy doing the most (you find yourself spending the most time doing), and which one you do best. There is probably one interest you have that you just LOVE doing and you could do it all day - this is a good initial guidepost to an eventual main career.

In terms of thinking about money and job availability etc, it's never too early to think about it - and it's a question you really need to answer for yourself. Would you mind doing a job that you didn't like that much for a stable income, or would you rather be earning less, but doing something you love? There isn't a right answer for everyone.

Also, if you end up making the wrong decision, it's not the end of the world. Another secret to finding what you really want is to try loads of different things and see if you like it.

One last thing I'll say is that, you don't always have to do exactly what teachers and career services tell you to do. One thing they don't teach you in school is that there are A LOT LOT more alternatives than simply going for the most employable (safest) major and getting straight into a job. Some people go to college/university and then start their own businesses or take some time out to travel and find themselves. Some people might not even go to college and get into an internship/apprenticeship scheme.

Now that the "economy is bad", there are literally LOADS of opportunities out there, and these opportunities will keep growing and growing. So don't be afraid to dream big (if you decide you want to) follow your heart