When I draw a circle, it looks like a Pac-man or a pie with a piece taken out.

This happens in Inkscape if you accidentally move one of the nodes on the circle. To fix it, select your circle and look for the icons at the top of your screen -- click the one I've circled here:

Inkscape Tip: If you have to fix this issue every time you draw a circle, go to Preferences > Shapes > Circles and make sure the setting is on "Last used style."