What is the first thing that happens to the Jewish people when they arrive at Birkenau in Night?

Men and women were sorted when the Jews first arrived at the camp.

When they first arrive at Birkenau, a subcamp of the Auschwitz concentration camp, the prisoners can smell burning human flesh.  This is particularly disturbing because of the fit that Mrs. Schächter has had in the train car about fire.  It seems to have been a premonition.  When they reach the camp, they find out that the Nazis are sorting people and burning them alive.

First, what possessions they have left are taken from them.  Then, the men and women are sorted and separated from each other.

"Men to the left! Women to the right!"

Eight words spoken quietly, indifferently, without emotion. Eight simple, short words. Yet that was the moment when I left my mother. (Ch. 3) 

With this moment, Elie Wiesel was separated from his mother and his sisters forever.  He went with his father.  Of course, he didn’t realize it at the time that he would never see his mother and sisters again. 

An inmate asked Wiesel how old he was.  He told him that he was fifteen. 

"No. You're eighteen."

"But I'm not," I said. "I'm fifteen."

"Fool. Listen to what I say."

Then he asked my father, who answered:

"I'm fifty."

"No." The man now sounded angry. "Not fifty. You're forty.

Do you hear? Eighteen and forty." (Ch. 3) 

This was lifesaving advice.  If Elie Wiesel was older and his father was younger, they would both be more useful to work.  That was all the Nazis cared about.  If you were old enough or young enough to work, and strong enough to work, you would be spared.  If you were weak, into the ovens you would go. 

When Elie Wiesel and his father arrived at Birkenau, it was 1944.  The war had been going on for some time at that point.  The goal by then was to ramp up the efforts to kill as many Jews as possible as fast as possible, as long as they were not useful for slave labor.