I just got a Yashica Mat-124, and after putting the first roll of film
through it today I realized either I'm doing something really wrong or
my camera's broken.

This is my first experience with medium format, I've used 35mm up until
now (except for a Holga, but that doesn't count). I'm having trouble
focusing. No matter how much I twist the focus knob on my Yashica I
can't see a difference in focus or sharpness on the viewfinder on most
things - I could see a difference when I shot a picture of a really
long and wide hallway, but the other 11 frames on this roll of film I
just assumed that since it looked sharp on my viewfinder it would come
out like that on my negative. That wasn't the case when I developed it,
everything was way, way out of focus.

What's the trick to focusing a TLR? I'm excited about using this
camera, I'm just a little confused with how focusing works since it's
obviously not the same as it is on my SLR's.