They sell the cables on the Apple Store for the simplest solution. Personally I use one of these, which adds all the effects onto the signal then passes it to the Mac via USB.
It's good because it has both a guitar input for if you are using software amp simulations in garageband, logic etc. and an XLR for if you want to mic up your own amp for your own sound. It's also cheap as chips!
1 more thing....if you ever wanted to get serious about recording (i.e above garageband), this interface supports Pro Tools M-Powered.