Tiresias (or Teiresias if you prefer) is the aged blind prophet whose appearance turns the plot in all Greek tragedies set in Thebes. He always tells the truth about the past, present and future - but the characters who need to pay attention are too distracted to listen. Too late do Oedipus (in Oedipus Tyrannus) and Creon (in Antigone) realise that the old man's words were accurate. Only in Euripides' Bacchae does he have a different role - Euripides seems to be satirising his role in Sophocles' plays, and he's a pompous buffon dispensing religiosity rather than wisdom. As frequently in Greek myth, there are two conflicting stories (both recorded by Apollodorus) telling us why Tiresias became blind, and how he became the religious consultant to the rulers of Thebes - for seven generations, it would seem. He appeares to retain some of his power even after death: in the Odyssey (Book 11, the nekuia or Book of the Dead) he is conjured up by Odysseus.
The more prosaic - he saw a goddess naked: never advisable (compare the fate of Actaeon). Out in the countryside with his mother, young Tiresias saw Athena bathing nude in a pool. His reactions are not recorded, but his punishment was swift and severe - he was struck blind, to ensure he would never again see what man was not intended to see. But having lost his eyesight, he was given a special gift - to be able to understand the language of the birds (and thus to foretell the future).
The other version - (first in Hesiod, and the one fancied by Ovid in Metamorphoses) is more exciting. Out as before in the country, near Mount Kyllene in the Peloponnese, he came upon a pair of snakes lustfully intertwined. He hit the copulating couple a smart blow with his stick - presumably striking a blow for animal decency. But Hera was not pleased: as the sensuous seductress of Zeus, she heartily approved of sex - even for the lower creatures. His punishment was cruel - the worst a man could imagine. He was transformed into a woman, in mind as well as body. But some time later (after seeing the copulating snakes again, but this time alllowing them their pleasure) he was released from his sentence, and permitted to resume his masculinity. All could then have been well, but Tiresias was drawn into an argument between Hera and her husband Zeus. A common area for marital discussion - who has more pleasure in sex - the man or the woman? Hera had been clever enough to let Zeus believe that men were superior in this as in everything else. But it was decided to check with Tiresias - as only he had known what it was actually like in the two roles. As a dastardly man, he revealed woman's greatest secret: on a scale of ten, she gets nine parts of the pleasure to his one. Hera was furious, and instantly struck him blind - Zeus couldn't do anything to stop her - but he did give Tiresias the gift of second sight.
Not surprisingly, from then on Tiresias lived the live of a reclusive ascetic - emerging only when there was a crisis in Thebes that needed his gifts. Not that the rulers were always appreciative - Oedipus in Sophocles' Oedipus Tyrannus mocks his blindness and curses him - although we, the audience, know that what he's told him is the truth. Creon in Antigone comes - on the Chorus' advice - to believe him, but loses his son and his wife for failing to unbury the living before burying the dead. In the Phoenician Women of Euripides (which is set in terms of plot just before Antigone) he tells Creon that Thebes will only survive against its seven attackers (the "Seven Against Thebes") if he sacrifices his son Menoeceus.
In Homer, the first time Tiresias appears, his life is already over - Odysseus has to consult him in the land of the dead. His advice is cryptic and unhelpful - Odysseus will be told by Circe herself what he really needs to know about his immediate future: Tiresias takes him forward to a time after he's returned home, when Odysseus will have to make mysterious journey inland carrying an oar. Thus we know halfway through the Odyssey that the happy ending we'd been expecting will not happen - after a brief reunion with Penelope he'll have to be off again.