Beat Red and go to Silph Co. and talk to Steven Stone. He's all the way on the right of the building next to a clerk's desk. ALSO: Top is grass, middle is fire, bottom is water.
You'll need to have met him in Vermillion City - talk to the Copycat in Saffron City, then head to the Pokemon Fan Club in Vermillion and get the PokeDoll from one of the members. You'll meet Steven outside after this. Taking the PokeDoll back to Copycat gives you the Monorail Ticket, which lets you ride the Monorail between Saffron and Goldenrod. It's obsolete now that the Indigo Plateau is a stopover Fly point, but hey, it looks cool.