Playing in time with Piano

I am creating an application for ipad using objective-c in xcode in which the user has to play specific piano notes in time with a piece of audio (Metronome sound)

I am fairly new to apple development and would like advice on how to code this.

I already have a piano that plays notes and a stave to display notes played.

So I would like the app to show a tick when the user plays the correct note at the correct time (say note C between seconds 3 and 4 of a 20 second piece of audio), and a cross if the wrong note is played, or played out of the specified time frame.

Any point in the right direction would be greatly appreciated

Thank you