Nothing to worry about in 99% of the US, North Korea doesn't have many nukes or even missiles that can get to the mainland US. They are also smaller A bombs not the big H bombs like we have. They might make us bleed with a first strike but they won't survive the retaliation. If they strike first with a nuke, the global response will be the end of their existence.
Kim Jon Un wants an excuse for a conventional weapons skirmish so he can show his people that he defended them against the evil US and declare their military might even louder. He doesn't want a full-out war.