niasimpson Hi there. It's great to hear that you've bought a phone from giffgaff ☺️.
If your current SIM card fits into your new phone, please continue to use that. The SIM provided with the phone is just a spare, just in case your existing SIM didn't fit or if you weren't using giffgaff and wanted to activate it.
As for phone loan payments, your first monthly payment will be taken one month after your phone delivery date.