Here is a pretty decent article discussing how to set up the iPad up for audio recording...
External Microphones for iPhone 4, iPad and iPod Touch Audio input | Life is a Prayer.com
Also, I'm not sure if Garage Band is the best app for what you're trying to do. I've read a couple of positive posts about these apps..
MultiTrack DAW for iPhone, iPod touch, and iPad on the iTunes App Store
StudioTrack for iPad on the iTunes App Store
This company sells a mic specifically made for the iPad...
I'm sure there's other apps, equipment and more options so others folks may have some additional advice.
Welcome to the forum and hope that helps!