Creation Kit
The toolset released February 2012 for modders, based on the new Creation Engine and built with usability in mind. It can be downloaded via Steam.
 Skyrim Workshop
The official destination for mods, it should prove a one stop shop for most. However, there is the alternative option of the Skyrim Nexus (see below).
 Tutorials & documentation
 Skyrim Nexus
The Skyrim Nexus is the former source of choice for the modding community, as it was been with Morrowind and Oblivion. Here you should can find plenty of useful mods, news and more, and of course, it's where you can submit your own mods.
 Official Skyrim forum
The official Skyrim forum again proves a valuable source of information and discussion on modding Skyrim.