In Act One, Iago becomes enraged because Othello has selected another officer, Cassio, to be his lieutenant. This slight, along with his seeming pleasure at watching bad things happen, compels Iago to hatch a plan to bring down Othello and Cassio.
Iago is careful not to let on that he dislikes either Othello or Cassio, or that he is plotting his revenge. Othello is so trusting of Iago (he repeatedly calls him "honest Iago") that he readily plays into the villain's hands.
Iago's first plot is to reveal to Desdemona's father the secret elopement of his daughter to Othello. (See 1.2.50-100) Iago also uses the foolish and unsuspecting young Roderigo (who had been rejected by Desdemona) in his plot.