The original GarageBand has only one mode to edit notes... and that's a bunch of dots... Command-Click to make new ones... However, since you read music, if you want to write music the standard way... you need GarageBand 2, part of the latest iLife, which lets you edit in music notation. If you double click a virtual instrument track, a note button should be in the lower left... click it, and notate away.
I was SO happy when they put this feature in GarageBand 2, I HAD to buy it. Plus, GarageBand 2 also lets you "play" your computer keyboard as if it's a piano keyboard. It's pretty dang spiffy.