I'm new to GarageBand and am curious if I can import a iTunes song and get GarageBand to score each instrument. That is, depict the musical notes of each instrument.

Um No!

if only music was that easy. aye

you could try to cover the song. that is re-write the song using your own instruments within garageband.

you could try to buy midi data (the notes used or tablature for computers) for some of the hit songs these days. I dont know where to look for that though.

then you could emulate the instruments in garage band.

but no, garageband will not work out or write the song for you.

Garage Band can only generate a score from a midi file, there is no score option for audio.