Easy answer is: Default = C:\Program Files (x86)\Steam\steamapps\common\skyrim\data\
Detailed answer is: Well, if you installed Steam in a different directory, try Koos' suggestion (either type "Steam" or "Data" and pick/open that folder). Note that there may be more than one "Data" folder.
By the way, there should have been instructions on how to install your Skyrim Mod (perhaps using the Skyrim Mod Manager would be a better idea for you?).
I CAN'T FIND MY STEAM FOLDER!!!!!!!1 I wan't to install the Stylish Spy Skin! i'm on a laptop
have a look in C:\program files (on 32bit windows) or C:\program files(x86) (on 64bit windows).