The first symbol on the staff is the clef.
The treble or G clef is used by high-pitched instruments such as the violin, flute and trumpet, and some low-pitched instruments such as the guitar:

The bass or F clef is used by low-pitched instruments such as the double bass, cello and trombone:

When reading music, pay attention to the clef to identify notes correctly.
Soprano clef:

In the past it was used for soprano voice.
Mezzo soprano clef:

Used by the viola and in the past also by the alto voice.
Alto clef:

Tenor clef:

Used by low instruments such as the cello, bassoon and trombone in the high registers.
Baritone clef:

All notes shown are middle C.
See Reading Music in the Tutorials section.