You have to collect Mega Energy for that specific Pokémon. Follow the guide below to know how to get mega energy in POKÉMON GO
Mega Pokémon can be found in Mega Raids, take them on, and earn Mega Energy for that specific Pokémon. The sooner you defeat them, the more Mega Energy you’ll acquire.
You will need to do several raids of the same Pokémon to have enough energy to Mega Evolve, and most Mega Evolutions rotate in and out of raids.
Note that the Mega Evolutions won’t be available year-round. Make sure to stock up on any Mega Energy you want before the Pokémon leaves Mega Raids.
You can also get Mega Energy from some Field Research and Timed Research Tasks as rewards.
READ MORE: LIST OF MEGA POKÉMON AVAILABLE IN POKÉMON GO