"Play" guitar on Mac mini?

I write songs, and have always used a guitar to work out the melodies. However, my arthritis has made it almost impossible for me to make chords on the guitar. Is there a program that will work on my desktop (either iMac or Mac mini) to "play" guitar chords - preferably with a single key to make a chord?

Welcome to Mac-Forums..

Check out Garageband.

yea garageband works excellent for these kinds of things