To study calculus it is essential that you are able to breathe. Without that
ability you will soon die, and be unable to continue.
Beyond that, you will need some familiarity with two notions: the notion of a number, and that of a function.
Suppose I have forgotten everything I ever knew about numbers and functions?
Do not worry. We will review their properties.
And if I know everything there is to know about numbers and functions?
Then you understand calculus already and need not continue.
Before reminding yourself about numbers and functions you might ask the following questions.
What is calculus?
Why should I learn about it?