Instructions
Go to Route 207 -- located due north of Oreburgh City.
Walk into the tall grass, and open your item bag. Scroll over to the "Key Items" portion of the bag, and select the Poke Radar.
Run to the sections of grass that shake and sparkle. Larvitar inhabit the grassy spots that sparkle when you use the Poke Radar on Route 207.
Capture a Larvitar. Lower its health in a Pokemon battle, and toss a Pokeball at Larvitar to keep it.
Train Larvitar to level 30, and it will evolve into Pupitar.
Raise Pupitar's experience level to 55, and it will finally evolve into Tyranitar.