The melody (the part that you sing) is played on the upper manual with your right hand.
The chords (used to accompany the melody) are played on the lower manual with the left hand.
The bass pedals make very low notes, called bass notes. They are played with the toes of your left foot. (Take your shoes off.)
Volume Pedal
The volume pedal, (sometimes called the expression pedal) is used to control the loudness of the sound. The further you press the pedal down with your right foot, the louder the volume. Press the pedal down about halfway for practicing.