I want to seriously devote myself to studying mathematics, since I'm currently in the last year of high school and I want to pass the entrance exam to a solid university so I can study CS. I have the list of the things that will be on the entrance exam, as well as the entrance exam from the last year. Tried looking for some textbooks but I feel overwhelmed. I want to study specifically to pass the entrance exam and a bit more. I don't want to dedicate too much of my time to some complicated things which I can't use.

Any help? Do you know of any good literature that I should use? How much is the high school knowledge I already have valuable?