First off, I am a recent switcher (as of last month), and GarageBand is by far my favorite application. My primary reason for switching was to record and make music with ease.
Thus, the first thing I wanted to do was plug my Strat into my computer and press record. In order to do this you need an interface to the computer (M-Audio FastTrack, GuitarPlug, iMic, etc.). However, you can plug directly into the computer using a special cable that features a 1/4" plug for your guitar jack and an 1/8" plug that goes into your input jack (the one next to your headphone jack) on the computer. I use
Griffin's GarageBand Guitar Cable.
Once you have gotten this far, be sure to check your preferences to make sure you have the right input selected. Simply click Garageband > Preferences > Audio/MIDI tab. Make sure Built-in Input (or GuitarPlug, if that is the interface you are using -- it should be an option if it is plugged in your USB) is selected as your Audio Input. Then create a new track to record your guitar (Track > New Track, or Option+Command+N) and select Real Instrument. To see the track's info, select Track > Show Track Info (Command+I). In the Track Info pane to the right there should be a Monitor option. Simply select it so it reads "On" and you should be able to hear your guitar as you play. Some guitar layers are louder than others, so be sure to turn up your speakers or select a layer that's loud enough to make sure it is working properly. As a side note, make sure mic volume is turned all the way down (the option right above Monitor in the Track Info Pane) as a precaution to avoid any kind of feedback.
You may have known much of this, but maybe I stated something you may have missed. Be sure to check your audio preferences. By the way, I would invest in some decent speakers or headphones if possible, because GarageBand's built-in loops and effects are fantastic.
Hope this helps.
