Check out the "Music Creation
" portion of the Apple Store, under "Mac Accessories." I personally use the M-Audio MobilePre. It lets you plug the guitar and/or microphone or other instruments into it, and then you plug it into the USB port of the Mac. It gets its power over USB so it doesn't have to be plugged in to the wall. You can also plug it into external speakers. Other ones have MIDI if you have a keyboard, and there are several other similar devices like it.
If your Mac has a mic input, you could just get the right cable and plug the guitar right into that, but you'd have to crank up the gain because it would lack a pre-amp. It works OK though, if I recall, and it would be cheaper than getting a box. If you don't have the audio in, though, you'll have to get another box.