If you are studying a typical post 16 mathematics course in high school, and your teacher isn't an incompetent Luddite, then he will have urged you to get a Graphics Display Calculator. One of the best is the TI-Nspire CX, which is allowed in almost all exams where calculators are permitted.
Now you need to learn how to use it effectively. So, no matter what country you are in, or what maths syllabus you are following, this book is written for you. I have absolutely no doubt that the vast majority of students will find plenty in this book that will help them achieve higher exam grades.