Welcome to the giffgaff community 🙂
As this is your first "smart" phone,
you will have to quickly get to grips with it and how it uses mobile data.
Do you have WiFi at home?
WhatsApp uses data, rather than call minutes,
but giffgaff mobile data is expensive without a goodybag.
You will also have to probably change some settings
for each of your Apps, so check your data usage.
Background data access can be safely turned off for most Apps,
refresh when you want to, not when the phone, or App wants to!
Turn off "Turbo mode"
(this uses giffgaff mobile data, when the phone can't find WiFi).
Most "smart" phones use data whenever, whereever they can,
it is up to us to control it 🙂