Put 2 Pokemon in the Daycare (on route 34, south of Goldenrod City), one male, one female. They have to be the same species or be in the same egg group. Or put the Pokemon you want to breed with Ditto.
Leave them in the Daycare for a while and they may produce an egg. The old man will give you the egg. The Pokemon will be the pre-evovled form of the female or the pre-evovled form of the female/male you put with Ditto.
