Sit down at a keyboard and look at each key, then at the sheet music. Notice the two sets of lines. The set of lines on the bottom with a backward "C" and two dots after the "C" is called the bass clef, which you play on piano with your left hand.
Look at the set of two black keys in the middle of your keyboard. The first one on the left side has a white key immediately to its left. This white key is called middle C. The keys beginning immediately to the left of middle C are the bass clef keys and these are the keys you play with your left hand.
Count down--toward your left--eight keys. Your finger should be resting on another C key. Count back up from C to the middle C. The keys in between are D, E, F, G, A and B. These eight keys make up one octave or eight notes. You'll repeat the pattern of eight keys from C to C from the bottom note all the way up to the top note.
Study the sheet music and look at the notes on the bass clef. Middle C has a line through the note. Going down, the next note--B--rests on the top line of the staff. The A has the top line going through the note. G sits between the top and second lines and F has the second line going through it. Play these notes.
Put both hands on your keyboard and play two different C notes. You'll hear two different notes, one octave apart. Play only with your left hand on any note below middle C. As you go down the keyboard, the sound from the keys becomes progressively lower.