Source SDK 2013
Part 1: Getting Started
Part 1: Getting Started

The sp and mp folders contain singleplayer and multiplayer code respectively.
Decide whether you'd like to mod vanilla mod_hl2, or episodic mod_episodic.

Episodic Half-Life: 2 includes Magnusson, his strider buster, and the separate flashlight energy HUD et al.
Once you've picked the game you want to mod, right click and drag the folder into steamapps/sourcemods.
Your mod will appear in your games list.

It won't work just yet - in part 2 we'll compile the code needed to run the mod.