Instructions
Join Team Rocket
Start a new game and make Charmander your starting Pokemon. When you start a new game of Pokemon Fire Red, you will be given the choice to select one of three starter Pokemon--Squirtle, Bulbasaur, and Charmander. Charmander is the first part of the code to join Team Rocket, so select him. Increase Charmander to level 50 without actually evolving the Pokemon. Every time your Charmander is about to evolve to its next stage, press the "B" button to cancel its evolution.
Travel to Cerulean City and head towards Nugget Bridge. Cerulean City is the second town reached after defeating the Pokemon trainer at the first gym. You reach it by traveling through the wilderness outside the town you start the game in. There are five different Pokemon trainers at Nugget Bridge. Defeat each of the contenders so that you can join Team Rocket.
Agree to join Team Rocket. After you defeat the five Pokemon trainers, a man will ask you if you want to join Team Rocket. Enter the following button combination: A, B, A, B, R1, R1. A game prompt will appear asking whether you would like to continue. Say yes. The man will say thank you. You are now a member of Team Rocket. You will lose every single Pokemon you have; however, you will gain a level 5 Mew and a level 5 Mewtwo.