Νομίζω is "to think" in the sense of "have an opinion", as in "I think you're right" or "I think that she is pretty" (= it is my opinion that you are right or she is pretty).
Σκέφτομαι is "to think" in the sense of "be in thought, ponder, cogitate, work things through in your mind" -- for example, if you want to think about a question before responding, you'd be σκέφτομαι-ing, not νομίζω-ing.
I wondered why the exclamation mark. We're always thinking. The comments here make it clear that we use the expression when we are considering something, or trying to answer a question. Actually, I wouldn't say I'm thinking about it, but "I'll have to think about it". Sometimes meanings are "lost in translation", but sometimes we learn how other people "think".