How do you get the fire blow in light platinum?

You can't get the Fire Blow in Pokémon Light Platinum. It is not a move that exists in that game.

Fire Blow is a Fire-type move that was introduced in Generation V (Pokémon Black and White).

Pokémon Light Platinum is a fan-made hack based on Generation IV (Pokémon Diamond, Pearl, and Platinum). This means that it uses the movepool and mechanics from those games, which don't include Fire Blow.

If you're looking for a similar move in Pokémon Light Platinum, you might consider Flamethrower, which is a powerful Fire-type move that can be learned by various Pokémon.