if your keyboard works as a midi keyboard controller you have lots of options. i'm not even remotely familiar with garage band, but if you want your keyboard to produce a guitar sound before adding effects or post recording architecture, you need to get a VST. there are plenty of free ones available, but quite honestly dropping a few bucks on one will get you a much more dynamic situation, wherein not only the sound of a guitar are replicated but also the way a guitar makes sound. some of them include aspects like fret buzz, looseness of strings in drop tunings, great chord definition and strumming, the sound of the pick hitting the string... all that subtle stuff.
that's if you want to actually mimic a guitar. if you're happy with the sound of a distorted piano or horn or something all the power to you. just keep in mind that the more effects plugins you use the more your cpu has to deal with and the muddier the sound will be upon the finalization of your track. if you're going this route, i recommend finding the sound you're going to use as the base (by "base" i mean the foundation of this guitar sound you're looking for, not bass guitar), coming up with an arrangement of effects, writing down what you did or saving it as a template, then writing and recording the song without the effects.
it also depends what kind of guitar sound you're looking for. if you're after just a big, crunchy or otherwise distorted sound, plankton's suggestion will certainly work. but if you're after a classical or acoustic sound than you need to find a vst (vst stands for "virtual studio technology" or "virtual synthesizer technology", depending on who you talk to. it basically means a virtual - software-based - instrument or effect).
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