Apps use data in the background for various reasons, including notifications. By restricting background data you can save a lot.
The second thing that uses a lot of data is Sync. I opted to switch it off. If I want something synchronised, I do it manually. Samsung devices give you the option to turn Sync on or off when you swip from the top of the screen. If the option is not there, click the three blocks on the top right corner:
Apps update by default in the Play Store. Open Play Store and select the three horizontal stripes on the upper left hand side. Then choose settings. The second option should be 'auto update apps'. Select that option and select either 'do not auto update apps' or 'auto update on wifi only'.
Of course not all Android devices will have the exact same options to disable the usage of unnecessary data, but by searching the settings menu, you can find these options.