To get a program such as Excel to divide numbers or to carry out any other mathematical operation, you have to tell it what you want it to do using the correct instructions in the correct order.
These instructions are contained in a formula. The formula tells Excel, what data to use, the mathematical operation to carry out, such as division, and where to place the answer in your spreadsheet.
This may sound rather complicated but it isn't really. To find out how easy it is, read the short article on How to Divide in Excel. This article also has links to other basic operations, such as addition or division, and multiplication.