Go to Settings.
Click on Cellular Data.
See all the apps you have and the toggles across from them? (Are they called toggles?)
Tap the toggles so they are at the left and are not green. I would suggest turning off data on things that don't need internet and/or are not necissities. Such as games, contacts, calender, etc.
Well, I hope this saves you money on your data plan! Please comment on how this worked for you.