I think Roadkill nailed with the phrase "if the singer isn't very loud". Position the 58 about forehead level pointed down at the nose and have the singer project straight ahead, not up. How far away will require experimentation. Ideally, you probably need the capsule at least five inches away from the mouth to avoid too much proximity effect which will give you an overly boomy bass signal. Too far away though, and you'll have to boost your gain and the noise with it.
Coach your singer to sing out and project that voice. Sing from the diaphram and use proper breathing technique, and all that. Then your job is to set up the mic and the gain stages of your gear such that you don't get any clipping but a nice strong signal that won't require a lot of jacking up later. Go do that, then come back and we'll talk compression/gating. Happy recording!
-kp