How to Get a Black Charizard on Pokemon Go
Unlike regular evolving, which is permanent, Mega Evolving temporarily transforms your Pokémon, changing their appearance, improving their performance in battle through a significant boost in stats, and offering other unique benefits. You can tell if a Pokémon is eligible for Mega Evolution if you see a "Mega Evolve" button and "Mega Energy" on their summary page.
Mega Evolving a Pokémon may also change their type. For example, Mega Charizard X is a Fire and Dragon-type Pokémon, while Charizard is a Fire and Flying-type.
Note: Trainers can only have one Mega-Evolved Pokémon active at a time. Also, a Pokémon's Fast and Charged Attacks don't change when they're Mega-Evolved.
Collecting Mega Energy
Mega Evolving a Pokémon requires Mega Energy, a resource that you can collect in multiple ways. You can earn Mega Energy by defeating a Mega-Evolved Pokémon in a Raid Battle or completing specific research tasks. You can also occasionally earn Mega Energy by walking with your Buddy Pokémon if you have previously Mega Evolved a Pokémon in your Buddy Pokémon's Evolutionary line.
It's important to note that Mega Energy can only be used for one specific Pokémon species. For example, the Mega Energy listed on Charizard's summary page can only be used to Mega Evolve Charizard.
Joining Mega Raid Battles
Mega Raid Bosses are easy to identify in the Map View or Nearby screen. Just look for a Mega icon 
:
The amount of Mega Energy you'll receive from a Mega Raid Battle depends on how quickly you defeat the Raid Boss. Upon defeating a Mega Raid Boss, you'll see a Speed stat on the summary screen, which determines how much Mega Energy you earn.
How to Mega Evolve your Pokémon
When you've collected enough Mega Energy, go to an eligible Pokémon's summary page and tap "Mega Evolve."
While Mega Evolving a Pokémon for the first time costs a significant amount of Mega Energy, Mega Evolving that Pokémon subsequent times will cost less energy. However, note energy costs are only reduced for the specific Pokémon you Mega Evolve, not others of its species.
After they're Mega-Evolved, Mega-Evolved Pokémon will stay in their Mega-Evolved form for 8 hours, after which they will return to their regular form.
Note: Certain Pokémon like Charizard have multiple Mega forms. When Mega Evolving a Charizard, you can choose to evolve it into Mega Charizard X (a Fire and Dragon-type) or Mega Charizard Y (a Fire and Flying-type).
Once you've Mega Evolved a Charizard, the Mega Energy required for future Mega Evolutions will be reduced, but only for the form you chose. If you want to reduce the energy cost for the other form, you'll need to Mega Evolve it for the full cost first.
Tip: Mega-Evolved Pokémon can't be powered up while Mega-Evolved. Their boosted stats are calculated based on their stats in their normal form, so make sure you've powered up Pokémon you want to Mega Evolve as much as possible before Mega Evolving them.
Using a Mega-Evolved Pokémon
Trainers can use Mega-Evolved Pokémon in different several ways:
Raiding - In addition to having improved CP, attack, defense, and HP themselves, Mega-Evolved Pokémon give other Pokémon special bonuses when brought to a Raid Battle. While a Mega-Evolved Pokémon is on the battlefield, all Trainers' Pokémon receive an attack boost to all their attacks and gain an additional attack boost if their attack types match the Mega-Evolved Pokémon's type(s). However, these bonuses don't stack if there are multiple Mega-Evolved Pokémon battling simultaneously, and the highest possible attack boost will be applied.
In a Raid Lobby, Trainers with an active Mega-Evolved Pokémon have a Mega icon next to their avatar and an aura surrounding them:
Gym Battling - You can battle at Gyms with Mega-Evolved Pokémon, but note that you can't leave them at a Gym to defend it. The attack boost applied to Raid Battles also applies to Gym battles. If you're attacking a Gym with a Mega Pokémon, other Trainers attacking the Gym at the same time will receive a boost to all their attacks and an additional attack boost if their attack types match the Mega-Evolved Pokémon's type(s).
Battling Team GO Rocket - Use your Mega-Evolved Pokémon to battle Team GO Rocket grunts or leaders, including Giovanni.
Battling Friends - You can battle someone on your friends list with your Mega-Evolved Pokémon, but Mega-Evolved Pokémon can't be used in GO Battle League.
Make them your buddy - You can set a Mega-Evolved Pokémon as your active buddy. If you're Best Buddies, you'll gain your Best Buddy CP boost in addition to the Mega Evolution boosts.
Take a GO Snapshot - You can take GO Snapshots of Mega-Evolved Pokémon just as you can with regular Pokémon.
Catching other Pokémon - You can earn additional Pokémon Candies when you have an active Mega-Evolved Pokémon and you catch a Pokémon that shares a type with your Mega-Evolved Pokémon.
Mega Pokédex
Tap the button in the bottom right of the Pokédex and select Mega to view all the Mega-Evolved Pokémon you've seen and caught.
Note: Mega-Evolved Pokémon are only marked as caught once you've Mega Evolved them, not when you catch a Pokémon after a Mega Raid Battle. When you defeat a Mega Raid Boss, they turn back into their regular form before you encounter and catch them, so their Mega form only be marked as seen.
How to Get a Black Charizard on Pokemon Go
Source: https://niantic.helpshift.com/a/pokemon-go/?s=top-articles&f=mega-evolving-your-pokemon&l=en