Nikon d100, aperture

I don't own the mannual but I downloaded it online. It said to go to A, and turn the sub command turny thingy. I did that but its still blinking "f--" and I turned the turny thingy for 5 minutes non stop and it is still blinking "f--"

The reason I'm trying to chhange it is because every picture I take come out blurry and dark. When I turned on the flash and took a photo it was just a white photo. My friend told me it was probably the aperture. Correct?

Lock your lens to its minimum aperture. Use the ring on the rear of the lens.

You need to look further in your user manual. Look on page 188.

The blinking F-- means: No lens attached, or non-CPU lens attached. Attach a CPU lens (IX Nikkor
excluded) or set function dial to M and use lens aperture ring to set aperture.

Further information is found on pages 20, 76 and 82