Brush tool can be activated through the main menu -> Paint -> Brush -> ...
Or you can click at the following button on the Painting Tools panel:
(you can open this panel through the main menu -> Paint -> Show Paint Tools).
You can adjust the size of the brush (in pixels), the feathering value (in pixels) and select its type:
1.Solid brush;
2.Gradient brush;
3.Spray brush;
4.Textured brush;
5.Centered texture;
6.Textured spray;
7.Image brush.
Warning! The image layer must be selected in the layers list in order to use brush tool.