The brush tool is one of the most fundamental tools in Photoshop. Photoshop documents are canvases, and there only so many ways with which you can interact with it. The concept of the brush tool underlies a large part of all other interactions. Knowledge of the brush tool serves as the core principle with which you can add, subtract, or change things in a Photoshop document.
This tutorial covers the following aspects of the brush tool: the diameter, color palette, brush presets & images, hardness & softness, and opacity & flow.
The diameter is a numerical value that increases or decreases the size of the paintbrush. The smaller the brush size, precise your brush strokes. It can be accessed through right clicking or increased and decreased with the bracket keys, [ and ].
Clicking on the foreground color in the tool bar once will bring up the color palette menu. With the long, thin, multicolored, rectangular strip on the right (1), choose a color and hue by clicking or dragging along the vertical strip.
After you are satisfied with this certain hue, you can choose the saturation and lightness with the large square box on the left (2) by, again, clicking on a specific area or clicking and dragging your mouse around.
Brush Presets & Images
Photoshop has presets for different brush shapes in the brush palette. Accessed through the drop down brush menu or right clicking on the document with the brush tool selected, you are able choose different types of brushes that Photoshop has to offer. This changes the way the brush tool leaves marks on your document (e.g. you can create stars, maple leaves, etc).
Hardness & Softness
Accessed through the same drop down brush menu or right clicking, there is a sliding option for hardness with a numerical percentage value from 0% to 100%. The higher this value that harder your brush's edges will be, and the lower it is, the softer. The sharper your brush is (the higher the percentage), the less it will blend in with the surrounding area.
Opacity & Flow
On the brush menu at the top, there are two numerical percentage values for opacity and flow respectively. If you click on the drop down option for either, a value slider will appear, and it allows you to change your brush's opacity or flow.
Opacity refers to how much your base layers' will show through, with 0% meaning your brush will leave no mark (it will be see-through and your base layer is completely visible) and 100% meaning it will mark over and cover the layer below.
Flow refers to the rate at which your brush "shoots" paint out when you hold down your mouse and drag across the canvas. You can imagine the brush tool as a hose; the lower the percentage value for flow, the slower the paint will "flow" out of it, and the more interrupted or discontinuous (like a sprinkler) your brush image will leave its mark. The higher it is, the more continuous the flow of paint will be.