(for my Mom)who play piano/guitar with mac garage band?

I just confused about garage band software. There are two types tutorial lessons. One is piano, other one is guitar. My mom want to study the piano tutorial lessons from garage band, so should we buy a usb piano keyboard from apple store? Because i see apple store sells piano keyboard, but they told me it is not for garage band? little confused .....

You need a keyboard with a midi interface or a line out. Midi would be better.