Pauline,
Don't ever be embarrassed to ask. We all had the same kind of questions when we started out.
Open the Fill and Stroke Box by clicking the icon at the top of your screen.
Attachment:
Capture fill and stroke.PNG
You'll see Opacity at the bottom of the box. You can just slide the bar to increase or decrease.
Attachment:
Capture fill and stroke box.PNG
I hope this helps.