Fill in at least two cells, then copy them down. Excel will follow the trend. It will recognize if you want to increment your number for example. When you copy only the first cell, Excel does not know what you are up to.
Second case: if your first cell contains formula referring to other cells, it is about absolute referencing. This means that all copied cells are referring to the same (other) cells, which you probably don't want. If this is the case, in the first cell, remove dollar signs ($) from (other)cell reference, and copy formula down.

what happens when you calculate (press F9) ?

Drag down either copies either increments the dragged cell.
The default behavior may depend on some options, check them.

While dragging, hold the CONTROL key down to do the alternate behavior.

As it apppears quite hard to explain ;-)
I suggest you look for some videos.