Recording good guitar tracks is very simple in Garage Band it is one of the staples that gave Garage Band their name of fame. ON the AR16 all you have to do is plug in your 1/4in. guitar cable into the jack at the top of channels 1 or 2 (You cannot use guitars on every channel only on 1 and 2). Once you plug it in there is a small square button right below the input jacks that has a guitar on it once you push it it the setup work is done.
Now what you need to do is set whether you want the guitar to record in mono or stereo and to set the EQ which can sometimes be a little tricky with guitar tracks so the best way to do it is just to listen to it and to play around with it. Now all you have to do is to open up Garage Band and select the correct input channel and start recording ! For more info you can see the video below.