How to stop using data on my Iphone?

I just got a txt saying i've used up 50% of my data allowance for the billing cycle. I've barely been on the internet or anything on my Iphone, when I first got the phone I was on the internet and stuff all the time and never got a message, i'm thinking I accidently turned something on or off and now it's using data like crazy. What can I do to fix this? I know I should call Verizon but i'm at work at the moment and can't and i'm not even on the account i'm just looking to see if anyone knows so I can fix this real quick, thank you!

Added (1). Turn on my Wi-Fi even if i'm not connected or can't connect?

Turn on your Wi-Fi don't use 3g/4g for internet surfing.

You need to clear out yout caches… Should be able to do this under settings

Turn on the Wi-Fi