I began playing with Garage Band and I decided it is like a musical Mr Potato Head (do they still make that? If not then people younger than me might not know what I'm talking about), because all you have to do is drag loops to the track area, and they automatically change to the key and tempo you've specified, so they more or less automatically sound good together.
You can also play on an onscreen keyboard, which is terribly awkward, or record into the program using a microphone or plugging an instrument into the computer.
Anyway, I put together a little piece of music, about 1.5 minutes and 1-2 MB, that I am pretty happy with as a first attempt:
http://www369.pair.com/jonesqn3/gb/song.mp3