So, you just bought ARMA 2 Combined Operations off the steam sale and you are now looking to play Day Z.
Here are some helpful guides to get you started:
Installing Day Z mod: Updated DayZ Installation (Steam/Retail/Mixed/Beta/Troubleshooting) - With [OCB]Magnet - YouTube
Short video guides of basic game elements: DayZ GuideZ - YouTube
Other helpful stuff: Collection of Dayz "tips" threads for your convenience. : dayz
OwnedCore Day Z Group on Steam: Steam Community :: Group :: Ownedcore DayZ