Mewtwo is a legendary Pokémon that was introduced in Generation I (Pokémon Red, Blue, and Yellow) and wasn't available in FireRed or LeafGreen.
However, you can catch Mewtwo in Pokémon Yellow, using the following steps:
1. Obtain the National Pokédex: You need to first have the National Pokédex to access the location where you can find Mewtwo. This is given to you by Professor Oak after catching 150 Pokémon.
2. Travel to Cerulean Cave: Cerulean Cave is located in the northernmost portion of the Kanto region. It's a very challenging cave with many powerful trainers and wild Pokémon.
3. Find Mewtwo: Mewtwo is located deep within Cerulean Cave. It's not easy to find, and you'll need to explore the cave thoroughly.
4. Prepare for a tough battle: Mewtwo is one of the most powerful Pokémon in the game, so make sure your team is leveled up and prepared for a tough battle. It is a Psychic-type, meaning that it is weak to Ghost and Bug-type attacks.
5. Catch Mewtwo: You can catch Mewtwo like any other Pokémon by using Poké Balls and weakening it. It is recommended to use Ultra Balls and potentially even Master Balls for a better chance.
Let me know if you have any other questions about catching Pokémon in FireRed or other Pokémon games!