For this you need a Ditto and a pokemon to breed with (It has to be a female), if you don't have a ditto you can catch one by first beating the elite four then going to the giant chasm. Put the Ditto and the breeding pokemon (example: Zoroark) in your party. Then go to the day-care center and give the person your Ditto and you breeding pokemon. Go outside and walk/run/cycle around for a minute then talk to the man outside the day-care center, he will say that he found an egg (If he says your pokemon arn't playing with each other then it won't work with that breeding pokemon). Walk around for a little bit then the pokemon will hatch into the lowest evelution of your breeding pokemon (Example: If your breeding pokemon was a Zoroark then your egg will hatch into a Zoroa).