create a selection with the circle selection tool(hold on the square button in the floating toolbar and the circle tool will appear)-in a new layer, making sure that you have the shift button pressed as you do it (this will ensure that it will be a perfect circle).
Once you have your selection click on the Paint bucket tool, select black as the colour(at bottom of toolbar-Foreground colour) and click inside the selection that you made.
Then hit ctrl+D or remove selection at the top of the page, to remove the selection and leave you with your circle.
Because you put it in a new layer you can move it around your document with the arrow tool.