There are many ways of copying down. With long ranges I like to use the
keyboard for a lot of the operations, as it gives more control over
positioning, but here is a very quick method if you have data in the
column to the left of the column you want to copy down (i.e. in column
B in your case).

Position the cursor on the cell with the formula in (C1). If you look
closely, you will see a small black square in the bottom right corner
of the thick border around the cell (this is called the fill handle) -
just double-click this and the formula will be copied down.

I assume that you put the formula into column C because you already
have something in column B, so this method will work. If column B is
empty, then cut and paste the formula into B1 and then double-click the
fill handle.

Hope this helps.

Pete