Some have said that with the MBP configured correctly, one can jack-in with just the guitar. One must also consider if your guitar has active or passive pickups. If passive, you'll need to feed the guitar into a pre-amp first to generate a line level that can then be input to the mac. If you have active pickups, the signal should be hot enough to drive the mac input without a pre-amp.
This is not correct. A guitar regardless of active/passive pickups is equal to the microphones used with computers. Both have weak signals. The mic jack is designed to accept devices with weak signals. The thing you do not ever want to do is amplify the signal that is plugged into the mic jack of a computer. At the least you will end up with clipping and a useless sound.
BTW, the "active" aspect of certain guitar pickups is designed to
modify sound from the strings not so much
amplify it. The output signal is still relatively weak when it reaches an amplifier. The active pickups in no way replace a guitar amplifier.
The only time you might want to use a preamp with a computer is if you use high-end headphones. The output at the headphone jack would need to be boosted to get the robust sound they are capable of producing. The typical signal from the headphone jack is not enough to provide more than a relatively flat audio landscape.
As stated in an earlier post all you need is a 1/4" to 1/8" adapter to fit a standard guitar cable into the mic jack to play the guitar through the soundcard of the computer. It is better to use an audio interface for several reasons, one being that the mic jack is not sturdy enough to handle the bulk of the adapters if you do this often. An audio interface has sturdy jacks like those on an actual guitar amp.
If you are using an interface the key to getting sound through Garageband with a guitar is to tell GB to listen with the internal card not the interface. It is unlike amp software and I have no idea why GB wants it this way but it will not work selecting the interface as the input. I have several versions of amp software. All of those need to be set up with the interface as the input. Most do not have an option to select separate input and output devices so everything comes and goes through the interface. Yet GB only picks up the guitar signal if you tell it to look for the internal system source instead of the interface. You do have the option of selecting something else as the output, either the interface or the computer headphone jack.