We'll start off by showing you how to create the most basic geometry — the brush object. Brushes are the basic building block for all levels built in the Source Engine.
To build your first level geometry, select the Block Tool, from the list of icons on the left side of the Hammer window. The tools are selected by single-left-clicking on the icon.
Select the Block Tool by left-clicking on it.
In the Top 2D View (the upper-right window pane), click and drag to create a box, as shown in this image.
Now that you've set the shape of the brush, we're now ready to create it.
Right-mouse click in the center of the brush to get the Create Object menu, then left-click the menu to create the brush.
Click the Selection Tool. The Selection Tool is used to select different objects in the map, as well as move and resize them.
Now we'll increase the height of the brush we've made.
Click and drag the square edge handle in the Side 2D View (the lower-right window pane).
The cursor should change to the move handle cursor when you have the correct handle.
Drag the height so that it is 128, as shown in this image. If you make a mistake, simply grab the handle and try again.
Creating a Camera
Next we'll create a Camera, which will help us navigate in the 3D View to see what we've created. Select the Camera Tool from the left side of the Hammer window.
Now you're ready to create the Camera.
In the Top 2D View, click and drag to create the Camera, as shown in this image.
You can use the Magnify Tool to change the zoom level in the 2D viewports. First select the Magnify Tool from the icon bar. Now left-click in a 2D viewport to increase the magnification, and right-click to zoom back out.
When you're done with the Magnify Tool, switch back to the Selection Tool.
Hint: The wheel on the mouse can also be used to change the zoom level in all of the viewports.
Now onto Creating a Room!