The moment Odysseus encounters the sirens is when he displays heroism. He is a hero through his leadership, wit, and obedience.
Odysseus knows that the sirens are not something his crew and he can just hope to make through. He has to form a plan and direct it out. So what he does is he gathers his crew, lines them up, and stuffs their ears with beeswax. That mean he had to have ordered men, who were probably really anxious after being at sea for so long, gained their trust and got them to listen to him no questions asked. This clearly shows he is a leader because a leader is one who has influence over a group of people. The way he organizes them and execute his plan effectively is an example of heroism.