How to answer "how's it going man?"

I'm very confused.

It's not just you; this is a common source of consternation even for people who've lived here their whole lives. Many people treat "How's it going?" as equivalent to "Hello", and consider "How's it going?" to be an acceptable reply to "How's it going?"; many others treat it as equivalent to "How are you?", and expect a reply along the lines of "Good; you?" or "Fine, thanks" or whatnot. (Either way, it's a pretty empty greeting; they're almost never expecting an actual explanation of how you are doing, unless the "it" is referring to something specific: "I hear you've been working in the construction industry. How's it going these days?")

Sometimes you can tell by the intonation; but often not.

A few generalities:

• Even someone who treats "How's it going?" as "Hello" will never mind if you respond with something like "Good; you?"; so that's generally a safe reply.
• Use of "How's it going?" to mean "Hello" is more common among younger men than among other groups.
• "Hi, how's it going?" is almost always an actual question, expecting a reply along the lines of "Fine" or "Good".

Both of your greetings are clearly very informal, and so do not require formal (or even terribly grammatical) responses. You could try,

Pretty good. How about you?

Can't complain. How've you been?

Hey, what's up?

Note that the last response isn't actually an answer, but in the circumstances that doesn't matter.

And all of this assumes a normal conversational tone and milieu. If you're sitting on the sidewalk with blood pouring down your face after having been mugged, "How you doing, man?" is probably more serious, and you can ask the questioner to call 911.