You don't. You create an instance and populate it with primitive data provided by the user (and I doubt you'd ask the user to manually enter serialized data like you can from a file).
So simply ask for the nextDouble off of the scanner, and instantiate the Circle with that provided double:
Circle c = new Circle(scan.nextDouble());. That will give you your circle so you can operate on the instance in the following output statements. It doesn't have any error handling in it, but I assume this is for an assignment (in which case error handling has not likely been covered)?