How to find out my data plan and how much data I have left?

I just get an iPhone and I got the smallest data plan I could. I really don't want to go over and I want to know how much data I have left! I've gone to settings, general, and usage to where it brings up sent and received and I don't understand all of that? Can someone help?

I have an app on my iphone and it tells me all the information about my plan and data usage. Find out if your phone company has its own app and if not just call and they'll let you know.