What's the best thing to do next after completing the biology library on khan academy?

Hi, I'm only a little older than you and I'd recommend you look at the chemistry library next, as in 10th grade you'll probably be dealing with chemistry next. However, this isn't true for all schools. I would ask your counselor or consult your schedule for next year if you already have it and double-check what your next science course will be. There is also a physics library if that turns out to be your next subject covered in school.