You won't be able to format the system disk from within, as it were. You have to boot from an alternative location - Windows installation media or some other bootable device so that the drive you want to format is not the drive you have booted from.
Formatting alone won't necessarily remove your sensitive data however - you need something that actually overwrites all sectors on the disk.
I would use DBAN:
https://dban.org/ - there's a good tutorial here:
https://www.lifewire.com/how-to-eras...g-dban-2619148
Note that DBAN (free version) won't work on SSDs, apparently.
SD