You need to use the famous Fill Handle of a cell. It is at the lower right corner of a cell. When you click on a cell you will find a lower right corner as a small dark square. Thats the Fill Handle. 

Hover on the Fill Handle and it will turn into Cross-Hair. Then Click on it, keep the left mouse button pressed and drag it to the cell range where you want to auto-fill.

Depending on the type of data in the first cell, of which you are using the fill handle, range selected will get auto-filled.

For example:

1. If the first cell has a number, lets say 5, then the rest of the range will be filled with 5.

2. If you select first two cells having numbers as 1 and 2, and use the fill handle of the two cell-range, then the rest of the cells will be filled with incremental numbers, i.e. 3, 4, 5, 6, etc.

3. If your first cell is a date, the remaining cell in the range will be filled with dates with increments of a day. 

4. If your first cell is an alpha numberic number, lets say Prod001, then the rest of the cells will be incremented by 1, i.e. Prod002, Prod 003, etc

You can play with this option and find your way out.

Now this is not all. Once you have dragged the fill handle to the desired cell range. At the end of the lower right corner of the range, you will find Auto-Fill options icon. Hover on it and click on the drop down black arrow, you will get options to change the auto-fill method.

So in example 3 above, if you have auto filled dates in a range, you can click on the auto-fill icons drop down arrow to find various options to fill, e.g. by months, by weeks, by year.

Try this out and you can do wonders.

My online course covers this and a whole lot more about data handling, data representation and data analysis, by using real life Excel files a company uses. Contact me if interested.