You cant, Only you can use a Game Shark.
if you have an AR for the gba
go to gameshark.com-buy gameshark
i dont know but u can find cheat codes at www.cheatcc.com
On an action replay
If its Pokemon Platinum, then you need the Action Replay which you can purchase at Gamestop. It's a slot that you put in your DS and you load it and put the cheat codes in.
First get a gameshark then look online for codes after that put the gameshark in the leafgreen game then type in codes. Also the codes can get you Pokemon and items instead of looking and looking you can get them easily. Most gamesharks are $15 to $20. Hope this helps ;)
Go to Cheats -> Cheat list... -> Gameshark... and then type in the code.
No.
action replay
GameShark SP, probably for Pokemon FireRed/LeafGreen
You must first have a cheat device for example Action Replay, then you can put the codes on that
put some cheat codes and their names