Volume varies from object to object, in geometry you memorize a lot of formulas for different volumes, and in Calculus you learn how to implement those memorized formulas in order to find more complex volumes.
to remember cylender just remember that you can think of it as the area of a circle. ANd think of a bunch of rings stacked up on top of eachother then see how high the stack is and you can therefore say pi r^2 h :)