Instead of writing a sharp sign before every F note on the staff, it is easier to write just one sharp sign after each clef. This means that all the F notes on the staff are played as F# , even though there is no sharp sign written before them. This is called a key signature.
This is the key signature for the key of G major. It has one sharp sign after each clef.
The C major scale contains no sharps or flats, therefore the key signature for the key of C major contains no sharps or flats.
The song I Yi Yi Yi is in the key of G major. Notice the key signature and play all F notes as F#.
This is a well known song from Mexico.
Chopsticks is one of the most well known pieces of music for piano. If your hand is not big enough to stretch the octave in bar 7 you can play the whole song using two hands. This song is in the key of C major as indicated by the key signature (i.e., no sharps or flats). Play the melody of Chopsticks without a chord accompaniment.