As a 15 year old
You're planning ahead! That's great!
A lot of us ended up in the field by going to school for it. Graphic Design is a field of study in a lot of colleges and universities. Some of them are BA (Bachelor of Arts) degrees, while some are BFA (Bachelor of Fine Arts). The latter tends to be more of a full fledged art school where you'd be exposed to all sorts of visual arts during your time.
There your studies would range from art history, to fine arts classes (painting, life drawing, printmaking, ceramics, sculpture, etc) and then core graphic design classes like fundamentals of design, color theory, typography, photography, illustration etc. During this time you'll learn to explore your skills, take critiques, and give critiques.
During your time at school, you'll end up building a student portfolio. This is what you'll ultimate value as a graduate. While a degree is certainly important, in this field it's mostly about your work.
Of course, there are graphic designers that never went to school for this at all, either. If they have a body of work to show, then that's their portfolio. It can be a bit tougher to go at it without a degree, but it's an option.
As for what you can ultimately do with a Graphic Design degree, there are many, many options.
Most folks end up working in either Graphic Design/Ad Agencies, or in in-house design teams in large companies. But there's many other jobs that would be applicable as well: art director at a publication, news industry, TV/Film titles, web design, UI design, trade show displays, signage companies, printing companies, etc.