Microphone not working (really quiet)

Hi!

My microphone has stopped recording well (on Garageband for instance) and my friends on Skype say they can't hear me very well (sometimes they can't hear me at all)

I have unchecked the "ambient noise reduction" box under preferences, but now, even though my mic actually picks up more of my voice now, there is a lot of ambient noise whenever I record something.

How do I get rid of the noise pollution?

It seems I can either have a clear but REALLY quiet sound or an ok- sound with a lot of noise pollution.

HELP! Thanks!