To be able to mega evolve, a Pokemon must have mega stone specific for it and the trainer must have a Mega Ring equipped. Take note that there are specific mega-evolutions available only in each version of the game. One good example is Mega Charizard and Mega Mewtwo that both come with the X and Y variant.
Mega Charizard X becomes a Fire/ Dragon type which significantly increases its Attack stat and gives the Tough Claws ability that further augments its attack boost. Mega Mewtwo X in the other hand becomes a Psychic / Fighting type that will have a considerable attack stat boost making it more than capable in dealing devastating physical and special attacks.
Mega Charizard Y still retains its Fire/Flying type but will boast a sleek aesthetic transformation, notably by having a larger wingspan. In this form, Mega Charizard Y’s SP attack stat gets a significant boost, further increasing the damage dealt by its Fire-type attacks. This is further augmented by its Drought ability that increases the power of Fire-type attacks. Mega Mewtwo Y gets a powerful boost in its SP attack stat further increases the damage of its special attacks.
It is a fact that not all pokemon can mega evolve. To undergo this special evolution, the pokemon should be holding a mega stone specific for it alone. For example, Lucario needs to hold a Lucarionite to be able to mega evolve, Charizard needs a Charizardite X or Y (depending on your game version), Mewtwo needs a Mewtwonite X or Y and so on. Some of these many stones are hidden in secret locations in Kalos, making them very valuable and rare. Most of the Mega Stones only become available post-game (after defeating the Elite Four for the first time) at a specific time of the day only. You can check out this Mega Stone Locations
page for more details.
A pokemon holding its unique mega stone is not enough to trigger a mega evolution. Players must use their Mega Rings to resonate with the stone to trigger this event. You'll obtain this item as part of the story in Shalour City.
This is a new feature in the game that will allow you to draw certain pokemon’s greater strength, in the form of Mega Evolution. Mega Evolved pokemon gains superior stats, abilities and even change types. However, this is just a temporary transformations that can only be triggered once per battle. This means even if the mega evolved Pokemon faints or switched, you cannot trigger any mega evolution in the same battle. Nevertheless, the strategic worth of these evolutions is undoubtedly invaluable and even enough to turn the tide of the battle. After the battle, the Pokemon will revert to its normal state.