Some people actually type a comma between numbers to depict a thousandth value, like 1,000! NEVER do this. If you enter a comma, the number becomes text and then cannot be used in any formulas. Instead type the number as is without any comma’s.
 
To add the comma’s, simply right-click on the required cells, go to Format Cells (ctrl + 1 is the keyboard shortcut for this), under the Number tab, select Number as the Category.
Then check on Use 1000 separator.
 
You can also select the no. of decimal places you want to have by default, and also select the way negative nos. in the data should look.
Do have a look at the Sample to make sure you selecting the correct options for the way you want your data to look like.
Use 1000 separator