Weaknesses of Mortal Kombat Characters?

I like to play MK9 online, but I almost always have trouble against Noob, Kung Lao, Raiden, and especially Ermac. Unfortunately, that's what almost everybody uses and they spam the hell out of those characters. What are some of the weaknesses I can exploit from them? I usually just hit random when I choose a character, but I'm best with Scorpion, Smoke, Quan Chi, and Sonya. So... yeah. Please and thank you!