Tyrion met Tysha on a lonely road, where she was being accosted by some men. Tyrion's brother, Jaime Lannister, drove off the men while Tyrion took care of her. She was a crofter's daughter, orphaned when her father died of an illness. Tyrion fell in love with her and married her, even though he was only thirteen years old.
When Lord Tywin Lannister found out, he had Jaime tell Tyrion that Tysha was a whore who had been paid to make Tyrion a man. Tywin then had her passed around among his guards to have sex with her; for each guard, he gave her a silver. To drive the lesson home, Tywin forced his son to be the last man, for whom Tysha was paid a gold coin, because Lannisters were worth more.
When Jaime rescued Tyrion from prison he confessed that Tysha was no whore. Tyrion realised that Tysha had been genuinely in love with him, and became furious with Jaime. He left his brother to find his own way out of the Red Keep. During his escape, he came to his father's rooms. Tyrion demanded his father tell him what had happened to Tysha. Tywin did not remember, simply stating that she went "wherever whores go". Tyrion slew him with a crossbow and escaped.