It's been a while since I studied for the U.S. History AP Exam, but my advice is the following: (1) use the questions (if available) at the end of each chapter as an initial guide on what are the most important takeaways
(2) try to jot down what you think are the most important ideas / facts from the chapter and review them with your teacher. Your teacher can help you figure out what you missed and help you prioritize important facts for the exam.
(3) learn what the AP U.S. History exam is actually going to test you on - topics and analytical abilities. I'd focus on learning and practicing what you'll be tested on and that can help take care of your concern on whether you are learning / taking notes on the most important sections of a chapter.
I hope this helps!