You will have to practice this on your own version of Excel.
Go to cell A1 and type:
1 
Point the mouse to the bottom right corner of cell A1 -
Your cursor should turn into a black cross-hair (+)
then click and hold the left button and drag to cell
A12. When cells A1 - A12 are highlighted, release the
mouse button.
You have now filled down the number 1 into cells A1-A12.
You can also create a series of numbers:
Go to cell B1 and type:
1
Go to cell B2 and type:
2
Now Highlight both cells B1 and B2, and fill down
(using bottom right corner of cell B2) to cell B12.
You should now see a series of numbers from 1 -12 in
cells B1-B12.
We can also fill down formulas. Let's go to cell C1 and write a formula to add cells A1 and B1.
In cell C1 type:
= A1 + B1
Fill down the formula through cell C12
You should see different values in cells C1-C12
Look at the edit line in several of the cells in column C - the formulas should change corresponding to the row that you are in.
If a particular reference needs to remain the same regardless of row. The reference should be proceeded by a $ sign.
Go to cell D1 and type:
= A1 + $B$1
Fill D1 down through D12
How is it different then column C? Look at several of the cells in column D. Notice that all cells in column D contain reference to cell B1.