Go to your system preferences and select the mouse.
De-select (that is uncheck the checkbox) for Move content in the direction of finger movement when scrolling or navigating.
Yes, it would be a lot easier if there was a button that says, Turn off the ridiculous reverse scrolling!, but there isn't. The logic behind this, by the way, to to pave the way for touch screens. On a touch screen, your finger moves the image in the reverse direction. Try pretending on the screen, it makes sense. But with the mouse, it makes no sense. Go ahead and set yours back. I did.