How can I get Garageband to stop playing BOTH the instrument sound AND the piano note at the same time?

I am trying to make a song on Garageband, and it is very frustrating to hear the piano notes when really all I want to hear is the software instrument. It wasn't doing this a moment ago, but now it is and I can't change it. It will play the two sounds -- the piano note and the software instrument note -- at the same time. I am using a Casio keyboard that is plugged into my computer. The sounds are both coming from the computer.