There are only seven letters used for notes in music. They are:
These notes are known as the musical alphabet.
As you go through the letters A B C D E F G, the sounds of the notes get higher. If you want to write music with each note getting lower than the one before it, you have to use the musical alphabet backwards - G F E D C B A. Practice saying the musical alphabet both forwards and backwards. Think of going forwards as climbing up and going backwards as climbing down.