Things You'll Need
Instructions
Insert the GameShark device into the Game Boy Advance. Insert "Pokemon FireRed." Power on the Game Boy Advance.
Select the "Codes" option and then select "Pokemon FireRed." If there is no entry for the game, select "New Game" and type "Pokemon FireRed."
Enter the Master Code if you have not already entered it. Select "New Code" and name the code "Master" or simply "M." Type "000014D1 000A" on the first line and "10044EC8 0007" on the second. Save the code and then turn on the code.
Create another new code. Name the code "Celebi." On the first line, enter "17543C48 E65E0B97," and on the second line, enter "4AEC27E8 A5FF1540." Save this code, turn it on and start the game.
Prepare to catch Celebi. Buy Ultra Balls and Potions at a Poke Mart. Arrange your team. Celebi's types are psychic and grass, so put a fire-type Pokemon in the first team slot. Do not use a fighting-type Pokemon, since they are very weak to psychic powers.
Walk around in any spot where you normally find wild Pokemon. When Celebi appears, reduce its health as low as possible without killing it. Use an attack that paralyzes Celebi for a better chance at catching it.
Throw Ultra Balls at Celebi. Celebi is very likely to break out of most of the Ultra Balls you throw, since it is a legendary Pokemon. If Celebi cannot break out of the Ultra Ball, it joins your team.