How does the King of Norway respond to his Nephew's plot against Denmark? What is young Fortinbras new plan of action?

You can find the answer to this in Act II, Scene 2.  It is
found in the passages where Voltemand is reporting to King Claudius the results of his
visit to Norway.

He says that the King of Norway was upset
to hear of Fortinbras's plans.  When he confirmed that Fortinbras was trying this, he
arrested him and made him swear that he would never attack
Denmark.

Instead of attacking Denmark, Fortinbras agrees to
take the soldiers he has gathered and use them to go attack Poland.  The King of Norway
asks Claudius to let Fortinbras and his troops pass through Denmark on their way to
Poland.