Right click on the cell and choose "Format Cells" from the menu.
It should open up a new window with the tab called "Number" selected by default. In this tab, select "Number" and then put in "2" for decimal places.
Click OK and now everything you put in the cell will be formatted for decimals the way you would like.
The help section on Excel is one of its best features actually, and you can find quick solutions to a lot of questions like these as you use it by going to the help menu.