How do you catch a staru in Pokemon FireRed?

You can't catch a Staru in Pokemon FireRed! Staru is a Generation II Pokémon (introduced in Gold and Silver). It was not available in the original generation games like FireRed.

You can find Staru in Pokémon Gold, Silver, Crystal, HeartGold, and SoulSilver.

If you're looking for a similar Water-type Pokémon in FireRed, you can try catching Staryu, which is also a starfish-like Pokémon.