how do i find out what books i have to buy for college after i have choosen all my classes already?

book sales are part of the college's income, so for that reason and that reason alone, they deliberately wait until the last minute to let you know, that way, you don't have time to order the book on-line for 70% to 80% less than they sell it for. I've seen books sell for $100 plus in a college book store, that I could buy on-line for $7.99, college's make millions on book sales

Usually, if you go to the college book store they will have lists of what professors require which books.