When I start modeling, whenever I press S to scale a selection, it would scale around the center of the selection. Somewhere along the line, I must have accidentally issued a command where everything is now scaling around the cursor. How did I do this and how do I get out of it? 
This is what you’re looking for. Here you can select the center of your trasformations (staling-rotating). The first option (selected object) transforms arund the selected object center (it you have multiple object selected, the one considerated qill be the one with the clearer borders. The econd option (individual centers) will scale or rotate each object around it’s own center. The third option (cursor), will consider the cursor position as the center of your transformation. The fourth option (Median point, is the one you’re looking for, and the last one will consider the bounding box center.
Ooh, thank you so much! That’s the problem. Thank you!