You can start from so many places but I prefer to do that what I did. Start to choice your emulator(S). I've choiced Dolphin and Dolwin. Then start to download demos. That are not gc isos but little games. You should find it easy.
Then try to run them on that emulator(s). If there are working it is time to try real games ( ). If you have your own gamecube and games rip them ( use google to find solution "how to do it") or if you don't have find them to download. You should know what to do now.