How do you get the pokeradar in Pokemon FireRed?

You can't get the Poké Radar in Pokémon FireRed.

The Poké Radar is a feature introduced in Pokémon Diamond, Pearl, and Platinum, and later appearing in Black, White, Black 2, and White 2.

FireRed is a remake of the original Pokémon Red and Blue, and does not include features from later games.