In Google Spreadsheet the default format for a cell is set as Normal. When you enter text in a cell the normal format would automatically align the content to left and similarly if the content was a number then aligns it to right. Let us say you have the following set of data with a column containing both decimal and normal numbers.
In order to change the format as Decimal for Average, select values entered for Average then click Format menu.
Under Format menu, navigate to Number and select 2 Decimal from the sub menu list. This would change the format as Decimal and the values would be displayed as shown below.
In the above examples the decimal numbers entered were all 2 decimals so we were able to select 2 Decimal format. But Google Spreadsheet also provides you with an option for defining custom decimal format.
Click Format menu and select Custom Decimals from the sub menu. In the following window enter the number of decimals
This would format the numbers entered in Average column as 1 decimal numbers.