Gosh, it would be easier to try to recall the times he told the truth—that's how often Iago lies in the play.
In general, though, there are lies of character and actual spoken lies. The lies of character are that he presents himself as someone who is on the side of Othello, Brabantio, Roderigo, Desdemona, Cassio, Emily…and he's not, ever. He's on his own side, the side of revenge.
As far as specific lies, he lies to Brabantio about Othello having stolen Desdemona's love (they did elope, but Othello did nothing else wrong), and about Othello currently having sex with Desdemona, even as they speak. He lies to Othello about where the handkerchief came from. He lies to Othello about what he overheard Cassio say about Desdemona, and about often seeing them together. He lies to Roderigo about Desdemona loving Cassio, etc.