To breed Pokémon you must have a female Pokémon of the Pokémon you want to breed, be it an evolved form or not. Some Pokémon such as legendaries and baby Pokémon cannot breed. The easiest way to breed Pokémon however lies in the hands of Ditto. If you breed Ditto with any Pokémon, male or female, you will gain it's lowest evolved form. Except in the case of Azurill and Wynaut,where to get Azurill one Marill must hold Sea Incense and in the case of Wobuffet one must be holding a Lax Incense. By the way, if any of you are too lazy to get Colosseum but end up challenging me at FR/LG and winning then you will obtain one legendary dog of your choosing.
Just look for a small sign saying '100 yards to Legendary Trainer J' or something along those lines in the East Midlands and you'll know you're in the right place.
EDIT: Has anyone else caught the Pokérus in R/S? Because everyone keeps telling me how rare it is but at least ten of my Pokémon has caught the Pokérus.