Remember the three T's.
Tear from the Top, Toward you.
The part in my left hand is what I want to keep so I also remember: Right/White
That means that when I tear with my right hand bringing that side toward me, the piece in my left will have an exposed edge. If it is designer paper, it will be White. Of course, if it is Stampin' Up! cardstock, the paper will be solid but you'll have that layered edge showing.
So if I tear with my left hand coming forward, the opposite happens and I don't have any white showing.
When tearing cardstock, you can always just flip it over. It is only with designer paper that you really have to pay attention to the direction you tear.