I had the real fisheye, nikon 10.5mm F/2.8 and even that was just a novelty that got old in about a week




If you are trying to take night shots with the D40 you are probably better off just using a tripod because It has terrible ISO noise.

Also if you can't even figure out the shutter speed on your camera you have a lot bigger things to worry about other than flash and fisheye lenses

First step read this:
Nikon D40 User's Guide