The Surface Inspector and the Texture Browser are the tools you will be using when texuring your maps. To use the Texture Browser, simply press T. To bring up the Surface Inspector for a surface, select a brush or surface and press S.
With the controls above you can either Fit a texture to fit on a brush, or use Axial to apply it 1:1.
The buttons on top row are used for brushes, the ones on the bottom row for patches. Use the CAP button for endcaps.
Here you can change how the texture is aligned on the brush or surface selected. You can shift (scroll) the texture, stretch (zoom) it, or simply rotate it. The value in the Step boxes control how much each value is incremented as the little arrow buttons are pressed.
This is the Folder List. Here you see all found texture folders listed, double-click a folder name and you will be able to see the contents of the folder to the right.
In the Texture List, all textures (and shaders) in the current texture folder are listed. Note that the border can be either green, white, both or none. White means the texture is a shader, green means the texture is used in the current map, and both green and white means the texture is a shader and currently in use.