Hi there,
Best place to control apps and data is in Data Manager. Anytime you add a new App. you should be checking in Data Manager to make sure you have it set to use the least amt. of data when at home (WiFi) or using WiFi. There is a check box for most prgs. there labeled: Restrict background data, you want that box checked. Directly underneath this box it says: Disable background data on mobile data network only. WiFi will be used if available. This shuts off the Mobile usage when at home (WiFi) or when you are connected to WiFi elsewhere.
If you do not have Data Manager on your phone you can dwnload it from the Google Play Store. Also***Be careful when Restricting Data*** in Data Manager. Some Apps MUST be fully on so that your phone works properly. I found this out through trial and error and was told by an ATT Rep that I had turned off too much, LOL!!! And so I was having problems with a few Apps. Android OS, Google PlayStore and a couple others MUST be on FULLY at ALL times.
Bottom line, call in and speak to a knowledgeable Rep. before you attempt this if you don't know what you are doing. But it will cut your Data usage down considerably once you know what you are doing and how to use Data Manager correctly. I hope this helps.