Talk to professor oak in pallet town, he will give you the instructions.
After you talked to him, you have to catch a 90+ IVs Charmander with a Timid, Modest, Adamant, Jolly or Impish nature and show it to him. After that you have to train that Charmander to level 100 with full EVs and show it to him again. Oak will challenge you for a battle and if you win you get the mega stone for Charizard Y.