generally people use it about a foot away although you could go farther. the sensitivity varies with distance.
cardioid mics are generally best for voices as due to the pickup pattern they can cut out some noises from other devices
using a mic stand and anti-vibration mount also helps and a pop filter will prevent breath noises.
if you still have trouble then you could try adjusting mic gain or placing the mic in a different area.
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its far from finished....
but if you click the "headphones" link in my signature there is some information you may want to read in the "what you need to know about microphones" section as well as a few recommendations listed in the professional microphones (desktop) category under reviews at the very end.
its a work in progress which will answer all of these sort of questions once complete.