If you want to see how much data you've used and which of your apps are using the most data, go to Settings>Cellular and scroll down to Cellular Data Usage and Use Cellular Data For.
Cellular Data Usage will show you how much data you've used in a given time period. This information does not reset automatically; if you want to keep track of this in order to prevent data overages, it's a good idea to set a monthly reminder to manually reset your data usage statistics on the first day of your billing cycle. You can reset the statistics by scrolling down to the bottom of the screen and tapping Reset Statistics.
Under Use Cellular Data For you will see an alphabetical list of all of your apps and how much data each one has used since the data usage statistics were last reset. You might be surprised to see that even apps you've never used before have used a small amount of data. You can prevent an app from using cellular data by toggling the slider next to it to the left. If you are concerned about data usage, it's a good idea to toggle off any app that you don't absolutely need to access while you're out and about. If you spend most of the day in Wi-Fi, that could be every app except weather, navigation, and fitness-tracking apps. You can always restore an app's cellular access temporarily or permanently if you find yourself wanting to use it away from Wi-Fi.
If you've really been hitting the data hard, you may want to turn cellular data off completely until your next billing cycle begins.