What you will probably do (I havent used Garage Band, but most audio editing software shares certain functions) is click within the waveform at the place you want to chop. Then look in your menus (Edit menu first) for a Split command. It might be called something else, but it will be similar to that. "Cut" won't be it. That will split the waveform at that point. You can then select the portion that you don't want and delete it. If you remove sound from the beginning, don't forget to slide the rest to the left or you won't have shortened it any.

Hope this helps.