Formatting Cells is a way of changing the view of the Cell. All Cells by default is set as general with no specific format being assigned. Although as you would notice Excel tries and figures out the best format like Dates.
To Get to Number Formatting you can right-click on the specified Cell, Row or column and Choose Format Cells… alternatively press CTRL + 1
The Percentage Category is the seventh of the explicit option you can choose from. This is a very basic format with only one option.
The option is to change how many decimal places to be used. The Cell will automatically add the Percentage sign after any numbers and align the numbers below one another.