On the ship, what does Athena tell Odysseus he is to do, and why?

Athena doesn't actually explain what Odysseus is to do until he lands on Ithaca.  Once there, and he knows who she is, she tells him that he is to remain a secret from his old friends/wife until he has a plan set to kill the suitors.  She also tells him that he needs to go to Eumaeus and he will be of help to him.  This is where she disguises him as an old beggar.