How do you get cheats for games that aren't even out yet?

We don't. 99 times out 100, when you see a cheat for a game that hasn't been released in your country yet, it's already been released in another one. Many console games are released in one region months (or sometimes years) before others, so it's likely that the cheat you're reading is based on that region's release.

What's a "Patch Code" (i.e. GameShark/Game Genie/Pro Action Replay/XPloder/Code Breaker code) and how do I use it?

A "Patch Code" for a game requires a "Patch Device" a.k.a. "Cheat Device", a cartridge or CD that allows you to overwrite certain bits of your console's memory with new values. The most commonly seen Patch Devices today are the GameShark and Pro Action Replay, which are available for most of the major game systems out today.

The means to use a patch code with a particular device are specific to that device, so you'll need to consult the manufacturer of that specific device for more information.