The brush is the main way of interacting with any painting and sculpting mode. While click & dragging in the 3D Viewport it will create a stroke and apply an effect depending on various brush settings.
It is highly recommended to use a Graphics Tablet for a better brush feel and additional features.
These are the most common hotkeys for controlling the brush.
After pressing these hotkeys, you can then either adjust the value interactively or by typing in numbers. Move the mouse right or left to increase/reduce the value (additionally with precision (Shift) and/or snapping (Ctrl) activated). Finally confirm (LMB, Return) or cancel (RMB, Esc).
You can also invert the brush direction/effect by holding Ctrl.