Photoshop provides two tools for drawing (painting) directly to the image: The brush tool and the pencil tool. Both tools work basically the same way — you left-click and drag your mouse in the image to draw. Painting uses the currently selected foreground colour.

The Brush Tool:

The brush tool paints with smooth edges. The options bar looks like this:

Settings:

Brush: The size of the brush.
Mode: The blending mode. For most work Normal will be the best option. Experiment with other modes to see what they do.
Opacity: Anything less than 100% will allow the underlying image to be seen.
Flow: Determines how quickly paint is applied. Lower setting produces lighter strokes.

The airbrush option

allows you to apply gradual tones to an image, like a traditional airbrush.

The pencil tool behaves much like the brush except that it has hard edges. The pencil tool options are the same as the brush tool, except:

There is no airbrush option.
There is an Auto Erase option. This paints the background colour over areas containing the foreground colour.

Note: To draw a straight line, click a starting point in the image, then hold down the Shift key and click an ending point.