The musical alphabet consists of 7 letters:
A B C D E F G
Music is written on a staff, which consists of 5 parallel lines between which there are 4 spaces.
The treble or "G" clef is placed at the beginning of each staff line. This clef indicates the position of the note G. (It is an old fashioned method of writing the letter G, with the center of the clef being written on the second staff line.)
The other lines and spaces on the staff are named as such:
Extra notes can be added by the use of short lines, called ledger lines.