Now look at the Toolbox panel. This contains a whole lot of tools at the top.

Below the tools is a small image showing fhe foreground and background colours.

Underneath that is an area that allows you to customise exactly what the tool selected does.

Select a tool using the mouse: that is place the mouse cursor over a tool, and click once. Note how the information at the bottom of this panel alters depending on which tool you select.

Then click on the paintbrush or the pen.