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 spider vanes in the eyepiece, the telescope is not focused.

As you move the focuser up, the image should get smaller until you reach a point where the shadow disappears. This image is now in focus. If you continue turning the focus knob and the shadow returns and image grows larger, you have passed focus and need to turn the knob in the opposite direction. If you want to make the focused image larger, you will need to use a higher power eyepiece (or zoom in if you use a zoom eyepiece).