Sorry - my cat managed to submit this before I'd finished!

Rather than a Shure SM58, try the SM57 - basically the 'loose' version which is great for instruments, but pops too much for vocals.

'Proximity effect' is the reason why the same microphone can sound totally different depending upon the distance between the sound source and the mic. This is because of the 'inverse square law' - sound pressure decreases by the inverse of the square of the distance as you move away from the mic.