How does Charizard evolve?

Charizard is the the final evolution of the line beginning with Charmander. It cannot evolve into another form (at least in the non-hacked games). It can, however, breed (at the Pokemon Daycare) and produce an egg which will give you access to an earlier evolution.

Charizard can also Mega Evolve into two forms: Mega Charizard X or Y using Charizardite X or Y. Mega evolution, unlike regular evolution, is temporary, only happening during battle.