It depends on the game and if there is a cheat for a gamecube game then look into the menu to find it or look on the internet to find out where to enter the cheat for the specific game.
To the best of my knowledge, the only way to cheat on the Gamecube is if you have a Action Replay device. It is possible that there could be other ways. I think though, that that is your best bet.
Cheats for gaming websites (webkinz, penguin club, millsberry)? Well, if you have a cheat code, you usually enter them wherever the giver of the codes says they work.
All the cheats are to get unlimted boost the cheat is : VROOOOMThe cheat to get all minigames is : MATTL66THe cheat to get all cars is : YAYCARSThe cheat to get all paint jobs is : R4MONE