Instructions
Start "Pokémon FireRed" in your Gameboy Advance. Select "New Game" or "Continue" if there is an existing save file where Squirtle was selected as the starter Pokémon. It is vitally important that Squirtle was chosen as the starter. If starting a new game, talk to Professor Oak and choose Squirtle when he offers you a Pokémon.
Defeat the Elite Four. This can only be attempted by visiting their headquarters after obtaining all eight gym badges. These gym badges are received after defeating the gym leader in each of the game's eight Pokémon gyms that are located in various towns throughout the game world.
Wander in any tall grass throughout the game's many regions. If you have defeated the Elite Four and selected Squirtle as your starter, Raikou may now appear. Its appearance is rare as well as random so it may take hours of wandering before it appears.
Capture Raikou by lowering its health as much as possible without defeating it and then throwing a Pokéball at it. Putting it to sleep or paralyzing it will make this process easier.