Click here to install the Source SDK. For future reference, or if that doesn't work:
- Open the Steam window.
- Go to the Library.
- Change the list to Tools.
- Find and double-click on the Source SDK item.
- Read over the dialog box and click Download and Install / Next to continue
- The Source SDK will then be downloaded to your computer via Steam.
Once Steam has finished downloading you can launch the SDK from the Tools tab. You can also create a desktop shortcut by right-clicking on the SDK in the Tools tab and clicking Create desktop shortcut from the context menu that appears.
When you first run the SDK it will spend a long time extracting its downloaded archive to loose files on your disk. This will not happen again until the SDK is next updated.
After that, you will be able to select a game and run any of the tools provided by the launcher. However, you will run into problems unless the game you want to work with is fully up to date. You can make sure it is by launching the game and quitting once you get to the main menu.
Where do I go from here?
If you want to learn about how to create content for the Source Engine in general, the main page of the Source SDK documentation is a good place to start.