You'll need something called an "audio interface". They can work either with USB or Firewire.
In this case Firewire is THE BEST way to go. Get a firewire interface and don't bother with USB.
You don't need to spend lots of money, even lower-priced interfaces will do what you need, and sound very good.
Most firewire interfaces for the Mac are now using something called the "CORE audio drivers" that are built directly into Mac OS X. This means you don't need to "install drivers" to get the interface working. You just plug it in, and the Mac will "see" it immediately.
Some (but not all) interfaces draw their power directly from the Firewire connection, so you don't have to use an external power supply block for them. Depends on the interface (not a big issue).
When you connect the interface, you will need to go to GarageBand and set the preferences so that GB sees the interface for your input. You'll also need to create "real audio" tracks (such as a "new basic track") on which to capture the audio. Be sure to bring up GB's "info" panel ("command-i", I believe) and get your mic inputs set properly.
By the way, there is a "digital audio" area right here on macrumors.com where you'd probably find folks with the best answers.
I started out with a Presonus Firebox which worked well for me, and am now using an Echo AudioFire8 which is even nicer. The AudioFire8 is probably more than you need - Echo also makes a couple of interfaces with fewer inputs (and nicer price tags!). They're good products.
As for mics, that's a whole 'nother story! If you don't have a mic yet, and would like something that sounds 'WAY better than the price tag suggests, check this out:
http://www.pssl.com/!YQbgOgolL!FWBFe3i49zMw!/MXL-MCA-SP-1-Studio-Condenser-Microphone-With-Clip-s
