Get the Essentials
Get Super Smash Bros. Melee. You can play on console (GameCube/Wii) or emulator (PC). For online matchmaking to work, make sure you have the correct version of the game, which is NTSC v1.02 (USA region). Also obtain an official GameCube controller. Generally speaking, you should try to get first party, Nintendo-made controllers over third party controllers (advanced guide). Rip your copy of the game and save it as an ISO for use with Slippi.