Ah trusty old Garageband.
iI use Garageband in all my songs, but i have never been able to make anything decent with a guitar. I think this actually might be a pre-set melody though... LOL.
You can still make a really cool song though with Garageband if you try hard enough.
Take a look at some of my songs, and let me knoe if you need any help.
I have helped 4 people so far. ^__*