Graphic / Web Design

I think at the age of 15(ish), trying to define your professional career is a bit too soon, especially if you're thinking of being a graphic designer. I say especially because I suspect if the thought of being a graphic designer crossed your mind, chances are you are more on the creative, artistic, slightly impulsive side. This might make it harder to really focus and become disciplined and organized enough to make a good living as a graphic designer (or any job in general). You might also enjoy drawing, painting, ceramics, animations, etc. as all those things appeal to your right side of the brain. Until you have grown a bit more into adulthood, I would advise to rephrase the phrase follow your passion to explore your passions.
Now the good news is that you are showing initiative in thinking about your future, unlike me when I was in 10th grade. The things I wish to have learned then is organizing skill and self-discipline. Graphic Designers generally don't make a lot of money, and no matter how good you are at one particular software or skillet, chances are someone else is just going to blow you out the water completely in terms of technical mastery. So, to get ahead of the game, I think the important things beside honing your artistic prowess is to improve on the weaknesses that many artistic individuals suffer fromlack of self-motivation, impulsiveness, ineffective communication & interpersonal skill, etc. And learn to be efficient in designing.