Problem with new microphone - T.Bone SC450usb

We, me and my boyfriend, have a problem as well..
The mic that we just bought - T.Bone SC450usb - doesn't work with Garageband...
When we plug it in, the sound from the mic AND the sound from the build in mic records at the same time... And the build in mic is even louder! And obviously that's not right. So does anyone know how to fix this?
Please, anything will help!
Thanks!!
DanSab