I created a plane, then dragged a .jpg on to it, but the texture appears flipped in both the x and y directions. To make appear like I thought it should, I had to set the tiling x and y to -1. The plane rotation was 0,0,0. Why is it reversing like this, and how am I supposed to do it properly?
You’re looking at it wrong. 
Seriously, nothing’s reversed, it’s all a matter of perspective as to what the “right” way is. If the default plane isn’t UV mapped to your liking, just import your own (which you should probably do anyway; the built-in primitives are primarily for prototyping and in most cases can’t really serve as actual game objects). Or just rotate the built-in plane.