At the very beginning of every song, there are three symbols that give information about where the notes will sound, about the beat or meter, and about the "Key" (whether there are sharps and flats). These three symbols are called the Clef, the Time Signature, and the Key Signature.
Clef
All violin music is written in treble clef. The notes within the staff are all above middle C (called middle C because it is the middle note on a piano keyboard)
Viola music is written primarily in alto clef, although advanced violists are expected to read treble clef also (and, even occasionally, bass clef!)
Bass Clef, played primarily by (in the string family) cellists and bassists, is also played by a pianist's left hand.
Treble Clef
Middle C in Treble
Clef
Alto Clef
Middle C in Alto
Clef
Bass Clef
Middle C in Bass Clef
Time Signature
A time signature tells the musician how many beats there are in a bar. The beats can be quarter notes, eighth notes or many other note values.
In this bar, there are 3 beats, indicated by the "3". The "4" underneath indicates that each beat is a quarter note.
In this bar, there are two beats, each one equaling a quarter note.
In this example, the "beat" is an 8th note, and each bar has 3 beats.
Key Signatures
A tune or song is often written based on one scale. For instance, a simple folk song might be "in the key of D", or, based on and using the notes of the D major scale. The D major scale has two sharps in it - F sharp and C sharp (Need to read about sharps and flats? Click here for more information).. A Key Signature, which usually is placed at the beginning of each line of music, indicates the sharps or flats in the scale or key that the song is written in.
D major Scale, written with no key signature.
D major Scale, written with key signature. When the sharps are written like this, in the key signature, the musician reads all F's and C's as F sharps and C sharps.
Keys and Key Signatures are a big topic. For more about this, click here.