Problem #1 is you are overloading your mic or your iMic with the input. Back off from the drums, engage the pad on the microphone if it has one, and/or get a better mic... however also realize that the iMic is built for line level sources and consumer (that is, taperecorder condenser) microphones.
A mixer or preamplifier that would handle the mic and send a line level signal to the iMic, which you could control the level, might help. Problem #2 as other people have stated, is that it's going to be very difficult to get a realistic recording of the drum kit with your present rig. Put a small mixer onto your shopping list. You can then start using more than one mic or input.
Using Garageband is not necessarily an indication that you are doing non-quality recording -- it is quite capable, especially v2
I suggest that you start reading some magazines like Recording, Sound on Sound, Electronic Musician, Keyboard, Future Music and Computer Music. They often have articles on microphone technique, recording acoustic instruments, and such. See if you can find Home Recording for Musicians by Craig Anderton.
Thanks
Trevor
CanadaRAM.com