Ah Charizard, the Original Fire Starter and nearly everyone's first pet dragon. Charizard's popularity is titanic, only matched by Pikachu, Sylveon and the pokégods themselves. As such, it was gifted with two Mega Evolutions which rocketed it into the spotlight.
Scorching Dragon
Charizard
Ability: Blaze/Drought
Item: Charizardite Y
-Flamethrower
-Air Slash/Focus Blast
-Solarbeam
-Tailwind/Dragon Pulse
Nature: Modest (+SATK, -ATK) or Timid (+SPE, -ATK)
EVs: 252 SATK/252 SPE/4 HP
Standard Mega Charizard Y. Mega Evolution is rather interesting in regards to the weather wars. While the slower weather pokémon being sent out at the same time "wins" Drought takes effect when Charizard Mega Evolves, so if Charizard is sent out at the same time as Tyranitar, it will "win" the weather war by Mega Evolving next turn (lest the Tyranitar also Mega Evolves...), allowing Charizard Y to strike at Tyranitar's now weaker SDEF.
In regards for moves, Flamethrower is reliable Sun-boosted STAB, blasting everything that isn't a resist or especially bulky. Air Slash is a secondary STAB for non-Rock Fire resists, while Focus Blast provides great coverage and strikes down Tyranitar with impunity. Solarbeam works great in the sun, roasting opposing Rock and Water types not named Tyranitar or Nihilego without much issue. Last move is up for preference. Tailwind patches up Charizard Y's decent Base 100 into something terrifying while providing double support with Sun. Dragon Pulse hits Dragons hard, which may be worth considering though a niche pick.
Other Options
Heatwave -If you are using Charizard Y in doubles, Heatwave is a godsend, roasting the opponents team quickly and without mercy.
Preferred Partners
Chlorophyll Sweepers such as Exeggutor, Sawsbuck, Venusaur and Victreebel all appreciate the sun, and will use it to run rampant over the opposing team, while providing STAB coverage against Water, Ground and Rock types, freeing up a slot on Charizard's moveset and having the potential to break through sponges that would otherwise trouble Charizard Y (Psyshock for Exeggutor, and the other 3 can go Physical). Sawsbuck can run Wild Charge to dispatch opposing Pelippers if it can catch them off guard. In turn, The Chlorophyll sweepers enjoy Mega Charizard Y's capability to tank sun boosted Fire attacks for them as well.
Tapu Koko -It easily dispatches most Water and Dragon types that trouble Charizard Y, and enjoys the fact that Charizard deals with most Ground types well due to their normally poor SDEF and Charizard's own immunity to Earthquake.