Using the GameShark
The GameShark is a top-loading device, meaning that you simply plug the game into the GameShark and then plug the GameShark into the Game Boy. The GameShark intercepts the data sent from the games programming and modifies it before displaying it on your Game Boy's screen. GameSharks can be purchased at nearly all video game stores. Once the GameShark has been properly connected, you can input the code you wish to activate.
Encountering Celebi
Connect your copy of "Pokemon Silver" to the GameShark and turn the Game Boy. Once the code screen pops up, enter "01FBEDD0." Start the game by pushing the "Start" button. Once you have selected your save file, walk around in any patch of grass or body of water. Anywhere you can encounter wild Pokemon, you can encounter Celebi as this code changes every wild Pokemon into Celebi. Once you have encountered a Celebi, you can catch it.
Catching Celebi
As Celebi is a Grass and Psychic-type Pokemon, it has several weaknesses, most notably Fire-, Ice- and Dark-type Pokemon. If you have any of those Pokemon, use them to weaken Celebi. Avoid using Fighting-, Water- or Ground-type Pokemon as Celebi will defeat them rather quickly. Once Celebi's health has been lowered to the red area, use any Pokeball to capture it. If the Pokeball shakes three times and then clicks shut, you will have caught Celebi.
GameShark Warnings
There have been reports of people losing their saved files when using a GameShark device. To prevent any accidental loss of data, do not overuse your GameShark on your copy of "Pokemon Silver."