If you select an object or several objects that can be filled then the fill style properties of the object will be displayed in the properties panel.
At the top of the fill styles properties panel you will see a collection of buttons as shown below which allow you to select the fill style.
You can apply 4 different fill styles.
The top part of the full style editor
Transparent or no fill
Solid Fill
Hatch Fill
Linear Fill
Radial Fill