To say hello you might say, "Hey, how's it going?" or "What's up?

To respond you might say, "I'm good" or "I'm alright" or "everything's good."

'Not bad' is a good reply too, but people usually follow it up with something. For example, "not bad, my son was sick last week but he's much better now" or " not bad, but yesterday was a crazy day." Something like that. Hope it helps!

I should add that this is my experience as someone who live in New York. As Gen Rugg mentioned, slang expressions are different from region to region.

I want to learn slang that can communicate with others..informal phrase

Are you interested in specifically English slang, or American?  If American, it really depends on the age group and the region of the country for the other person.