I'm trying to download SCP Containment Breach, but every time I try downloading it I get an error saying that fmod isn't there so I can't boot up the game.

Have you even tried, or read the mod's page? You need to download SCP: Containment Breach and unzip the SCP: PR files into the main game's folder.

I'm a bit new to having a computer so what are the PR files that must be unzipped. I've gotten so far as to extracting all the files and now when I click on the application the error screen comes up. I've looked up tutorials and I've done all that was required however it still does not work. (Also I don't know if you mean mod as in moderator or as in modification, like an add on, I'm just trying to download the base game .