You need a double end 3.5mm cable and a 3.5mm to 6.35mm adapter so you can connect your guitar to the mic jack in your computer, then you need to make your mic as default in the sound settings on your computer, you should then get a signal and hear your guitar through speakers and then I think you need to make your mic as input in amplitube and then you should be able to use it. A better (and more expensive) solution is to get an audio interface and connect your guitar to the interface with the guitar cable and set your audio interface input in amplitube