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.
Iago's plot at the beginning is to reveal to Desdemona's dad that she has eloped with Othello. But he has to make Desdemona's dad realize how bad that really is. Note all the beastial references to sex. Othello is a beast and Desdemona a goddess thereby making that union ugly and profane.
Othello is able to pacify the group though and so another plot has formed by the end of act one. He is going to use Cassio to make Othello jealous of Desdemona.
His purpose for doing this isn't just because he's was usurped for a high position - he also admits to being just an all around bad guy - then comes the kicker - it has been suspected that Othello slept with his wife! Ouch. He admits he isn't interested in verifying the information but rather he's going to run out and act as if it were true. He is portrayed as a devil and is a Christ-like figure. Note the way he comes out to greet the group who show up at the inn.