I would take all the mathematics you can, especially the pre-calculus and calculus course during your senior year if such a thing is offered by your mathematics department. You should take physics and chemistry, although biology is not required for astronomy. You should also take a typing course because astronomers do a lot of work with computers and writing. Also take english and make certain you can write clearly, formulate essays, spell, and logically present an issue based on well-reasoned arguments and evidence. You also need to become familiar with computers and master at least one language, preferably FORTRAN or 'C'. Don't be intimidated by this, but don't waste time learning BASIC. You need to learn a computer language, not just because astronomy is computer-intensive, but because computer programming is an important practical skill you might have to fall back upon if you do not get into graduate school, or even after you get your PhD!
As a proto-astronomer, your education will resemble that of any other physics major until you get into your junior-senior year in college, when you can take your first upper-level astrophysics courses.