Help with using a Nikon D-40 digital camera?

I have a school project due and i already emailed my teacher and he hasn't responded so i'll try here so maybe i can get a little bit done today. I'm having problems working the camera, when i select Manuel(M) the photo is too dark and sometimes nothing appears on the screen even though my F-Stop is at 4.2 and im in a well lit area,then when i select Speed(S) the photo again is too dark and when i select Aperture(A) everything is blurry. Could you maybe tell me how I am supposed to change the F-stop on M and S so my photos can come out and to change the speed on A so everything will come out in focus.