Zeilik and Gregory (https://www.amazon.com/Introductory-Astronomy-Astrophysics-Saunders-Sunburst/dp/0030062284) is pretty good if you don't have a background in the field. A more general introductory physics book would also be a good idea. I'd definitely recommend brushing up on calculus and getting a head start on differential equations wouldn't be a bad idea either.

Take a calculus book out from an academic library. An academic library is a library for a college or university.