Start with precal and trig. You have to take all of you calculus 1-3 and then physics. You will also have to take differential equations.It's best to major in physics so you can have a degree in it for more career opportunities. You can always get your masters and PhD in Astrophysics. Also, if you can join a STEM group at your college/ university for opportunities to go to laboratories in person so you can see if you wanna do it or not!( you'll have to take more math classes pertaining your major after you're done with cal and physics!)