Study basic mechanics and special relativity, then analytical mechanics. Then study calculus and differential equations. Then study cosmology and general relativity and tensor calculus. Then from there start quantum mechanics. From there you should have a pretty decent understanding of astrophysics and the fundamental concepts that permeate it
That being said astrophysics and high energy physics are gay and saturated with unattractive people, so do condensed matter and make money by treating semiconductors like the toys that they are
