The very hardest chapter in an introductory physics textbook is the first one. Students are suddenly, violently confronted with a seemingly unending barrage of new concepts and procedures. However, there is an underlying logic to it - and once you can see that, you will find that the rest of the textbook follows precisely the same logic. The goal of science, after all, is not to memorize a list of known facts, but rather to learn how to think like a scientist.