the average will go in Col C, and there will be nothing in C1:C19, because you want the average for the last 20 dates; until the 20th date there aren't 20 dates to average?
Assuming that to be so, enter the following in C20
=Average(B1:B20) and hit return
Then select C20 and hover the mouse over the bottom right of the cell, until it turns to a black cross. Left click and drag down the cells below and it will copy the formula into the cells you select.
Because Excel uses relative cell references, (by default), the formulas in each of the new cells will have the formula adjusted acordingly, i.e. cell C21 will contain the formula
C22 will have
and so on and so on....
Does that help?