Give it a bit of a search - we'ver covered this copious times before.
A line is is not a Mic in. You need to boost the signal of the microphone to line levels. You could do this with an external preamplifier, or mixer, or you could get an alternate interface like a Griffin iMic USB - or if you are doing more quality-critical work, check out the Digital Audio subforum for recommendations of digital audio interfaces and preamps.