It doesn't work like regular evolutions. Regular evolutions occur after battle when Pokemon level up. For Mega-Evolutions, you will need for them to hold a specific item unique to that Pokemon. So Charizard's should be Charizardnite.You will also need that Pokemon to get a Key Stone and a Mega Ring. Then, while IN-BATTLE, you will need to press a button and choose an attack. The Pokemon will mega-evolve AND attack on the same turn. As for charizard, after evolving, Charizard will have increased special attack AND have the ability drought. The downside is that you can only use 1 megalution every battle. But that keeps the game from being broken XD.
Hope this helped! :)