There are a few steps that need to be done before compiling scripts with Papyrus. Out of the box, a Skyrim installation isn't set up for any aspect of modding. Compiling scripts in particular requires additional steps above and beyond installing the Creation Kit.
Install TES V: Skyrim. You can't mod the game without it unfortunately. A legit Steam copy is considered necessary to get the Creation Kit.
Through Steam, [install the Creation Kit](http://www.creationkit.com/index.php?title=Category:Getting_Started#Installing the Creation Kit) following the instructions here. Even if you are just working in the scripting realm with Papyrus, the installation of this tool will include script libraries needed to compile just about anything.
Find and unpack the Scripts.rar archive into your installation directory, commonly C:\Program Files (x86)\Steam\SteamApps\common\Skyrim\Data. The result should be a Scripts directory like C:\Program Files (x86)\Steam\SteamApps\common\Skyrim\Data\Scripts.
Download the installer for Skyrim Script Extender (SKSE). Additionally, download the .zip source archive. Although the C++ source files will be unnecessary for most purposes, the SKSE archive also includes Papyrus source scripts. These are required to compile any mod that that uses SKSE functions like
Keyword Function GetKeyword(string asKeyword) native global. Open the archive and merge everything in Data\scripts back into your installation.
If you will be dealing with modding that requires a SkyUI "main menu", you will need to download SkyUI and unpack its scripts in the same way you unpacked the SKSE's.
Finally, enable logging and tracing in your configuration. It's necessary to see the messages produced by trace statements in scripts:
After the above steps you will be set up to compile scripts without hitting errors due to missing libraries and references. Follow the steps here to configure Notepad++ to have syntax highlighting for Papyrus and shortcuts to compile.