Who is the Choragos and what is his task in scene 1?

The Choragos is the "leader" of the Chorus.  He does most of the talking in the Chorus.  The rest of the Chorus will chant or repeat what he says, but he is clearly the leader.

In ancient Greek tragedy, the Choragos might very well have been the leading man, even top billing over Oedipus, as the Choragos was an actor who would have commanded respect.  The Choragos would have occupied the highest level on the stage and been on stage nearly the entire play.

In Sophocles' play Oedipus Rex, the Choragos is the voice of reason.  He introduces Tiresias and urges Oedipus to listen to him.  He also urges other characters to avoid extremes, to practice moderation, and see and hear the truth.