In Scene 1, the audience discovers from Roderigo and Iago's conversation that Iago hates Othello in part because Iago has been passed over by Othello for a position of power which was given instead to Cassio, someone with little military experience. Iago promises that he will "serve [his] turn upon [Othello]" (1.1.44). This conversation causes the reader to know how Iago will get his revenge upon Othello.
In the first scene, Iago also discloses that an elopement has just taken place. Othello, a black Moor, has secretly married Desdemona, an aristocratic, only child of a white Venetian noble. Iago plans to awaken Brabantio, Desdemona's father, by startling him out of bed and telling him the news that Brabantio will go after Othello.
Thus, the first conversation sets the stage for all the play's conflict.