Tyranitar isn't anywhere in the wild in Pokemon FireRed. You will need to evolve it from Pupitar. It is required to get a Larvitar first, however; it is in Seavault Cave and can evolve into Pupitar at level 30, then Pupitar into Tyranitar at level 55.
Larvatar evolves at level 30. It takes a little while, but in the long run, can be well worth it. Larvatar will evolve into a Pupitar at that level. In addition, Pupitar will evolve into a Tyranitar at level 55.