How do I make it so when the countdown gets to 0, something happens like the screen shaking?

Typically, a game will have a core loop, a loop that runs indefinitely which performs the basic operations of running intermissions, choosing an minigame, loading the minigame, spawning the players into the game, initiating the minigame's code, waiting until the end of the game, and repeating the process.

This core loop is pretty much a simple while true do loop. Screen shaking is something that isn't too difficult to do on a basic level, but on a more advanced level, can be rather complex. There are a number of screen shake modules available out there.