The first symbol on the staff is the clef.
There are several clefs. The more often used are the Treble Clef or G Clef:
The Treble Clef is used by high-pitched instruments such as the violin, flute, and trumpet, and by other lower-pitched instruments such as the guitar.
And the Bass Clef or F Clef:
The Bass Clef is used by low-pitched instruments like the double bass, cello, and the trombone.
When you are reading music, notice the clef in order to correctly identify the notes.
Notes shown are all middle C.
Mezzo soprano clef
See Reading Music in the Tutorials section.