You cant. You need to trade it from Ruby, sapphire or emerald version.
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Shroomish is not available to be caught in the wild in Pokemon FireRed, it must be traded from Ruby or Sapphire.
Yes, you can find Shroomish in Pokemon Ruby. They would be somewhere in Littleroot I think... If you find one, just look at pokédex where you can find them.
A Pokemon guide that you can find in the forest.
Piplup does not exist in Pokemon LeafGreen or FireRed.
Shroomish is rare in the Safari Zone in Pearl. Keep on looking there! The Great Marsh. There is no Safari Zone in Pearl.