I would check if it is april the first. Then I would disbelieve a lot. And after there would be undeniable proof that it is aliens and not some ploy of a warmongering nation, I would curse a lot, that those aliens have nothing better to do than coming to us and attack.
Maybe read about War of the Worlds, when it was a radio play in 1938 . That might give an idea what might happen. While the reactions were played up a bit, some people did believe the broadcast, as they turned in after the introduction and it was a lot of breaking news style.
https://en.wikipedia.org/wiki/The_War_of_the_Worlds_(1938_radio_drama)