You may want to display your composition with standard notes, clef signs, and so on. Select a software instrument region and open Track Editor by clicking the button at the lower-left corner of the screen. Click the notation view button (it has a musical note on it) in the lower-right corner of the editor’s header area and start composing.
You can print professional looking sheet music of your composition by choosing File→Print in the GarageBand menu.
Even though Apple makes it easy to use GarageBand, mixing or balancing all the parts of a song so that one track doesn’t drown out another can be difficult and takes practice. You can find various mixer controls and level meters under the Mixer.