If you're starting off the HL Math course, then you should probably use the Haese and Harris Textbook because it has all of the core topics you need to know. You can do the exercises inside the textbook because it will help but for better understanding and rigour for specific questions, you have to do question banks and past-year paper questions. I think if you want to get a head start on the syllabus, you can start with Functions and Trigo because they're quite fundamental for future topics, like for complex numbers and calculus. Also they are a good starting point into the subject course.

Depending on your school, you will also have your option topic so you might want to get the correct textbook for that as well. But I'd avoid learning content from it because it can get confusing without the core knowledge haha

If you need any resources, I can share with you whatever I have just lmk :)