Welcome there, stranger !
First question, why ? Why would I bother modifiying a Gamecube Game. It looks amazingly complex, and I can just buy better games nowadays, or wait for Nintendo to release official editor. If you think so, you are on the wrong page.
If, just like me, you miss the Gamecube, the Gamecube controller (the best controller that ever existed) the engine of a particular game, the practice you had with a particular game or even wish you could finish this adventure game that you love with your best friend, well, you will love game modding.For the skeptics, yes, you can do all this. Just look at the following videos :)
Playing on a beta track of Mario Kart Double Dash
Super Mario Sunshine multiplayer
Super Smash Bros Melee, with new levels (actually, it is Silly Melee)
Modding Games - How ?
Well. This is the hard part. The good news is that the older a console is, the more methods are available to hack games. Eggad Workshop is focusing on Super Mario Sunshine, Shiboo release a Mario Kart Double Dash track editor, Super Smash Bros melee has countless tools… Action replay codes can be thought as small mods through the game (though they do not need you to rebuild the game).
How to start ?
It depends on your skills ! In some cases, the tools are so easy to use that you just have to use what exists to achieve what you want. You can have a look at the following tutorial to enjoy a two player Sunshine game :)
If you want to dig dipper, I would recommend the following tutorial, to produce your own Action Replay codes (it does not require many tools, and you can achive amazing results in hours)
A bigger step would be to rebuild the isos. Basically, you go through the game files, change them, and launch the game. I will show how in a next article :)