Celebi is a special and rare Pokémon found in the Pokémon game "Heart Gold." You only find Celebi by completing a specific set of steps in the unique side quest as soon as you have gained access to the Ilex Forest. Once you reach that place in the game, gear up your finest Pokémon because you have to fight Celebi before capturing.
Go to Ilex Forest.
Go to the Shrine area in the centre of the forest.
Place a Master Pokeball onto the shrine.
Fight Celebi when it attacks you.
Reduce Celebi to a quarter of its natural health using fire abilities.
Use a Pokeball to capture it.