Give it the Gyaradosite Mega Stone and press the mega evolution button at the bottom of the move screen when you send it out. If you want to know where the Gyaradosite is in X, it's up the stairs in the South-East of Couriway Town. Also you need to battle your rival in Kiloude City, then go to Anistar City. When you speak to Professor Sycamore, you can now get certain Mega Stones at 8:00-9:00pm everyday. In ORAS, you have to pet the Poochyena in the Go Fish hut on route 123. If this is not clear, go look it up on Bulbapedia.