Why does there appear to be a black circle in the middle of my image when I look at a star or planet?

If you can see the shadow of the secondary mirror (black circle) and/or spider vanes while viewing through the eyepiece, the telescope is not focused.

Turn the focusing knob until the black shadow becomes smaller until you reach the point where the shadow disappears. The image should now be in focus. If you continue turning the focusing knob and the shadow returns and the image grows larger, you have passed focus and need to turn the focusing knob in the opposite direction. If you want to make the focused image larger, you will need to use a higher power eyepiece.