Ohoo scout21078, there's things need doin' first:
1. Fly to Lumiose City.
And go into the train station where Professor Sycamore will be waiting for you. The train station is located at the end of North Boulavard, just pass Route 16, the one on the end, leading to dendemille Town. He will give a ticket to go to Kiloude City
2. Battle the Battle Mansion
Take that train to Kiloude City, and fight the Battle Mansion atleast once. It is located in the centre of the city, behind the Pokemon Centre.
3. She is waiting for you...
Now if go to the far north of Kiloude City, Serena (or the boy depending on your gender) will be waiting to battle you. After defeating her, she will give you an Absolite and tell you that Professor Sycamore has made an amazing discovery to do with Mega-Evolution, and is waiting for you in Anistar City.
4. Fly to Anistar City
Professor Sycamore will be waiting for you infront of the sundial; talk to him and he will update your Mega-Ring so now your can find Mega Stones
Find them Mega Stones!
Between 8-9pm, Mega Stones can be found across the region, and will be the sparkling spots in the ground. In total, the are 26 mega stone to collect in total, with Mega Charizard and Mewtwo X and Y are version exclusive, however you will have already collected some mega stones already.
Hope this helped!