cannot hear guitar in garage band

I just got an imac and bought a cable (one end plugs into the electric guitar the other is a usb) and plugged into the imac. When I start recording I hear nothing and when I play back I hear nothing. I am not even sure it is recording anything other than dead air or perhaps the noise in the room. How can I tell that it recognizes my guitar? Do I need a different type of cable or an analog digital converter?