What does Nick learn from his first encounter with these people?

Concerning The Great Gatsby, you
should remember that Nick's first meeting with Tom and Daisy in the novel is not his
first meeting with them.  Daisy is his cousin and he went to school with Tom.  Jordan
Baker is the only person he meets that he hasn't met
before.

I'm assuming, by the way, that you're referring to
Nick's encounter with Daisy and Tom at their home, since you don't
specify. 

Also, Nick is so biased against Tom that he's
obviously made up his mind about him before he even enters his home on the
Egg.

That said, though, Nick's negative view of Tom is
reinforced by his first encounter with him.  Tom is cheating on his wife and takes phone
calls from his lover at home.  Daisy is in a miserable situation and has become
cynical.  And Jordan, the person he meets for the first time, is uppity and stuck on
herself (that's Nick's first impression, anyway, even though he swears that he doesn't
judge on first impressions).  And Daisy and Jordan both like to play games, so to
speak.  Daisy, perhaps as a defense mechanism, but Jordan because she likes the gossip
and the scandal.

Nick sums up his visit by saying that his
first thought is to call the cops. 

At the same time, of
course, beauty is present at the encounter--the home and the people are beautiful.  The
description of the scene during which Nick enters the house for the first time and the
women's dresses are floating with the blowing breeze, etc., is a special, though
deceptive, moment of beauty in the novel.