According to your description, your math foundation is relatively poor. There is no doubt that you should start with the textbook, without considering which part is difficult and which part is relatively simple. From beginning to end, make up bit by bit. To complete every exercise in the textbook, it is best to find an answer book and study it against it (I remember I have a textbook, and Wang Houxiong's textbook is completely interpreted. This book is good, but the questions above are quite difficult. If you don't understand, ask the teacher. In fact, the questions in the textbook are relatively simple. You'd better read the whole textbook in this way before you start to assist other reference books.
If you really want to find which part is simple, I personally think that probability statistics is the simplest, because these problems are relatively closer to life, and some methods of solid geometry are dead, mainly to establish a spatial coordinate system to solve, and if you master it, you will score. Trigonometric functions need to remember some formulas, but it is not too difficult. Inequalities and functions require high ability, while conic curves require high calculation, which is very troublesome. Let's come back later.
Come on, ha! I wish you learn math well!
If you have questions, you can continue to ask questions and seek adoption.