I'll try to explain Iago's complicated plot in easy-to-understand steps.
Step 1: Get Cassio, Othello's right-hand man fired, so that Othello no longer views him as honorable. (Iago does this by getting Cassio drunk and involved in a brawl.)
Step 2: Convince Cassio that the only way he can get Othello's trust and respect back is to plead his case before Othello's wife Desdemona so that she will talk to Othello.
Step 3: Arrange to be walking with Othello when they just "happen" to see Desdemona and Cassio talking quietly.
Step 4: Make suggestive comments to Othello about Cassio's way with the women and his relationship with Desdemona.
Step 5: Warn Othello to watch his wife closely (so that he will notice all the ways in which Iago plans to frame Desdemona and Cassio).
Step 6: Provide the "ocular proof" that Othello asks for. At the end of their conversation, Othello is not entirely convinced that Desdemona would be unfaithful to him; for all that Iago has provided so far are innuendos and lewd observations. The visual proof ends of being the handkerchief that Othello lovingly gave to Desdemona ending up in Cassio's hands.
I hope that this helps. Iago is the perfect two-faced character, and his plot to bring down Othello and Cassio involves master manipulation and numerous characters; so I know that it gets confusing in Acts 2 and 3.