All the C notes are located as the first white key to the left of the groups of two black keys. On many keyboards Middle-C as its name suggests is in the exact middle of the keyboard.
If you do not find a C in the middle you should take the next left of centre C key as Middle-C.
Middle-C is the most important reference point on the keyboard. To the left of Middle-C you play most of the bass-figures in Pop and Rock (with your left hand) and to the right of Middle-C you play melodies and chords (with the right hand) (This is not a hard and fast rule!)
