Obi-Wan Kenobi finds the clone army on Kamino in Star Wars: Episode II – Attack of the Clones.
Kamino is a remote ocean planet inhabited by the Kaminoans, who specialize in cloning. Obi-Wan travels there while investigating the bounty hunter Jango Fett, whose genetic template was used to create the army. The Kaminoans explain that Jedi Master Sifo-Dyas supposedly ordered the clones nearly ten years earlier for the Galactic Republic.
The discovery is initially puzzling because Kamino has been mysteriously erased from the Jedi Archives, and the Jedi Council did not know the army existed. Jango Fett is also raising an unaltered clone of himself on Kamino: Boba Fett.
A common mix-up is Geonosis, where the Clone Wars begin and the clone troopers first enter a major battle. Kamino is where the army is created and trained; Geonosis is where it is deployed against the Separatist droid forces. The Republic ultimately uses the clones as its military during the Clone Wars.