Tiresius the prophet tells odysseus he will get home if

Give sacrifice to Poseidon

Explanation:

When Odysseus visited the land of the spirits, he spoke with the Agamemnon's spirit and learned from the Theban prophet Tiresias how to soothe Poseidon's anger.

Odysseus wanted to talk to Tiresias, and they headed to the  River Acheron  in Hades where they offered sacrifices so they could talk to dead souls. Odysseus's mother, friend Elpenor and Achilles gave him various tips on how to overcome the obstacles that await him. Odysseus sacrificed a ram, and blood drew ghosts. He kept them at bay and demanded to speak to Tiresias.

Tiresias told Odysseus that, upon returning to Ithaca, he had to offer the sacrifice to Poseidon in order to calm him down. He also told him that he would die as a wise and old man if he did.