"Εσύ" is "you" in the nominative. The meaning of "σου" depends on its use in the sentence, can be either the possessive determiner or the weak form of the genitive case.
E.g. This is your father. Αυτός είναι ο πατέρας σου.
I'm telling you the truth. Σου λέω την αλήθεια.