"How's things?" or "How are things?"

Solution 1:

It is an idiomatic expression, fairly common here in the Midwestern USA, at least; it is equivalent to "How are things going?" or "How are you?"

Solution 2:

I've tried running this through my head to hear what I actually say, and what I wind up with is either "how are things" or "how's everything", both of which agree in number. However, I'd regard "how's things" as an acceptable alternative in informal speech, if it's generally used in your region. In a more formal context (a job interview, for example), I would avoid any of these constructs in favor of "how are you".