The play begins in medias res with a conversation between Iago and Roderigo. At first, we are not sure what or whom the two are discussing. We do know that Roderigo is clearly upset over a recent development for which he at first is blaming Iago.
I take it much unkindly,
That thou, Iago, who hast my purse
As if the strings were thine shouldst know of this.
Iago redirects Roderigo's anger away from himself and toward Othello--whom he does not name at first, by swearing that he hates Othello as well. As the act progresses we learn the reason for Roderigo's anger: Roderigo had been giving Iago money to help him win the heart of Desdemona, and she has eloped with Othello.
Iago's anger is twofold: he is angry with Othello because Othello promoted Cassio over him, choosing Cassio as his lieutenant rather than the more experienced Iago. At the end of the act, we learn that Iago suspects Othello of sleeping with his wife.
Iago and Roderigo go together to Brabantio to tell him that his daughter has eloped with Othello. Brabantio is furious. He is angry that his daughter slipped away during the night to get married; he is angry that she married a Moor and not one of the "curled darlings of her nation."
We see Othello at first through the angry perspective of these three characters. But Othello's entrance on stage readjusts our first impressions of him, and we find that Othello is actually honorable, confident, composed, and respected by his men and the Duke of Venice.