Well I mean I started when I still practically a toddler. 

My parents first took me to a planetarium when I was about 4, and I’d been obsessed with space ever since. That christmas my parents got me some space picture books, and as soon as I became a better reader, my mom started taking me to the public library to get more books. My dad got me a pretty decent telescope for my my/his birthday (at the time I think around $300?) and so by the time I was about 14 I had a pretty good wealth of knowledge about astronomy. When I got to high school and had the opportunity to pick my classes, I went and took the hardest math courses I could. I spent a semester in an Astronomy GenNet class (I don’t know if other schools have GenNet but it’s basically a dark room that you and about 10 others go into and a computer teaches you instead of a person). My senior year I took college level physics.

In college I started off with intro to astronomy and intro to astronomy observation classes. I suggest take as many major specific classes you can inn your first year. You get to meet your faculty advisors at a younger age, which is something you’re going to need to do eventually.

I skipped university algebra and precalc and went straight to calc, which is hard. If you do that, it will be a struggle. If you know you’re not up to the challenge, don’t do it.

I got into Quantitative Reasoning Physics (QR basically means it’s harder and you will cry more often) as well as QR Physics Lab (again, more crying).

Plan on going to grad school, too. There isn’t much market for Astrophysicists at a bachelors level. So with that in mind, do as many extracurriculars that have to do with your major as possible. Graduate schools will interview you and look at your resume, so make it pretty. The more of your free time you spend working with the subject you’re majoring in, the better you will look. EVEN BETTER start a major specific club. I started an astronomy club, both because there wasn’t one on campus, and because it’ll look damn good on my resume.

If there are major specific residence halls, jump on that. I’m in the Science and Technology Residence hall, which means I only live with other Sci/Tech majors. This will look good on a resume, too. 

Do volunteer hours. Throughout your college career, you will most likely have to do some number of volunteer hours, might as well do a bunch, put it on your resume, and look good doing it.

I hope that helps you!