To get Excel to perform a task, like division, 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 the spreadsheet.
To learn how to get Excel to divide numbers, read the article How to Divide in Excel. This article also has links to other basic operations, such as addition or subtraction, and multiplication.