Photoshop can be a great tool for creating drawings and paintings. Most people associate Photoshop with photos. Truth be told, Photoshop can be used to create more traditional looking art. I must specify that it may look like traditional art. The process is clearly different from creating traditional paintings and drawings. For one it's much cleaner. It is also much easier to correct mistakes. Conversely, finished Photoshop drawings and paintings can never replicate the look of an original painting or drawing.
Creating a sketch in Photoshop is as simple as picking a brush style and making marks. It is important to understand how to use the different attributes of the paintbrush tool. You will need to change the opacity and flow and perhaps adjust the pen pressure. Learning how to sketch in Photoshop will allow to create drawings that you can use for finished artwork by developing the image over several layers.
The following art lesson video shows you how to sketch using the paintbrush tool. In this video, I am using a Wacom tablet to loosely sketch a quick portrait. This video art lesson gives you a quick introduction to the use of the paintbrush tool in Photoshop CS4.