How can I save data volume // book cheap?

I have a mobile phone contract with Smartmobil. Announced monthly, with 3GB Internet. My Huawei used only 1 Gb every month. Now, since December, I have a new contract (24 months), which will run until the end of December 2019. The new contract gave me a new Samsung. Since I got the Samsung, however, more data is being used than before. I come every month about my 1.5 GB and unfortunately can't change the contract.

Does anyone have a hint? How can I save data volume? I'm both at home, as well as in college, as well as with friends always in the Wi-Fi.

I'm a bit desperate

The mobile data is always off when you do not need it at all or are in the WLAN. Close background apps and look in the settings for data consumption which app draws a lot of volume, then just note that you do not use them too often. Would always be what I do now

Always turn off your mobile Internet connection when you are not using it. When I'm at work, she is out. At break, it's the first thing I do with my phone, internet on, and off right at the end of the break. This also saves a lot of battery.
Basically do not download videos (Whatsapp etc.) or watch (Youtube etc.).
You can set in many apps, how to handle data. For example, you can set Whatsapp, the vides and images are downloaded automatically only via Wi-Fi.
You can check in your phone which app consumes how much data. If this is Yotube, you know what you need to do. (Use it only in the W-Lan).

It is actually instagram which consumes the most. I would not have thought

Instagram, many pictures in high resolution and sometimes videos. That literally breaks up data volume.