To tear cardstock hold it between your thumb and fore finger and use your other hand to tear the cs down and towards you and use your thumb on the hand holding the cs to sort of guide it. Don't just rip into it
You need a "controlled" tear or it will usually look a mess.
To distress I use sandpaper over white core cs or you can also lightly crumple cs and rub an ink pad over it or run the edge of an ink pad along the side of your cs.
Hope this makes sense, it's kind of hard to describe