Instructions
Go to Mount Silver, the mountain which bridges the gap between the Johto and Kanto regions.
Walk around inside the mountain's caves until you encounter either a wild Pupitar or a Larvitar.
Use Grass- or Water-type attacks to wear down the wild Pokemon's health points until its health bar is colored red, then catch it in a Pokeball.
Assign the Pupitar or Larvitar to your traveling party of Pokemon and use it in battles so it accumulates Experience Points and gains new levels. Larvitar will automatically evolve into Pupitar when it reaches Level 30, and Pupitar will evolve into Tyranitar when it reaches Level 55.