Some Internet plans charge you based on web usage instead of having an unlimited amount of usage. The amount you're charged is based on your plan, which is usually for a certain number of megabytes or gigabytes. Cell phone data plans work in the same way. You are given a certain number of megabytes a month to use online with your phone and if you go over you're charged extra. In order to make sure you don't go over you can calculate how many megabytes you use regularly.
Go to an online megabyte calculator like Web User, Bandwidth Calculator or Mobile Broadband Info.
Input the amount of time you spent on the Internet, the number of emails you sent and so on into the online calculator.
Click "Calculate" or "Submit" to see how many MB you use on a regular basis.