How to use less cellular data (iPhone)?

So my dad pays my phone bill and he said that I've been close to going over my usage and he'd have to pay an extra £40 if I did so I want to know how to turn off the settings so that I use less. I know it downloads stuff without the Wi-Fi but I don't know how to turn it off and all the other things that use up the cellular data.
I really only want to use the data for when I go on twitter when I'm away from the house. Should I just switch of the cellular data usage altogether and just put it on when I want to use twitter? But will I still get the notifications if I don't?

No. 1 Make sure you are connected to Wi-Fi when you are at home. Always.
No. 2 Go into your iPhone Settings--->iTunes&App Store---> Use Cellular Data. Make sure it's turned off.
No. 3 Don't watch YouTube or stream any video unless you are on Wi-Fi.
No. 4 Don't go on games when you're not on Wi-Fi.
No. 5 Go into your iPhone Settings---> General---> Background App Refresh---> turn off all that you really don't open that much.
No. 6 Go into your iPhone Settings---Cellular and turn off all of the apps that you don't want to use cellular data for (like for apps you stream music/video from especially).

Hope this helps.