The brush tool is the heart of digital painting. It can be a bit confusing, and this video will help familiarize you with the basic components like Opacity and Diameter. Interestingly, the way to make the brush tool more useful is to limit the functionality. In this video I explore the difference between my two favorite types of brushes:
Opacity Brushes use the pen pressure on your stylus to control the amount of pigment, and have a fixed diameter. Keyboard shortcuts are used to change the diameter.
Diameter brushes use the pen pressure to control the diameter, and have a fixed amount of pigment. This creates the effect of applying ink with a bristle brush -- you get a varied line weight, but a consistent intensity of ink.