First you need Steam. http://www.steampowered.com Then you need to either buy a copy of Half-Life 2 or The Orange Box to run the mod. This can be picked up retail for between 20 and 50 dollars. It can also be purchased directly from steam.
To install copy the “Virtual High School” folder (copy the folder that is in the same folder as the readme.txt file. Don’t copy the dated folder.) to your SourceMods folder in your steam directory.
ex: C:Program FilesValveSteamSteamappsSourcemodsVirtual High School
Restart steam and the game will show up under your games list if it was installed properly. It will show up as Virtual High School.
[ Edit: Sorry the download is no longer available. ]