Lots of astronauts have come from homes “in the middle of nowhere”. If this is truly something you want, then I would say absolutely begin now. Prioritize your studies and your physical fitness. Read books about astronauts and how they got to where they are. You need to understand, this is a lofty goal, but the fact that you’re only 15 and thinking about this is an excellent start. If you work hard, make good choices, and surround yourself with people that can help you, I believe you can make something great of yourself. There’s no better time to start on your dreams then right now!