how do I reformat a internal disk drive?

Assuming your device is a Mac then what processor is inside? Intel or M1?

How to start up from macOS Recovery

Restart your Mac and hold down command-R to boot into recovery mode. Once there you can launch Disk Utility to erase and reformat the internal storage. You can also reinstall macOS in recovery mode.

But before doing that please tell us why you want to reformat your internal drive. What problems are leading to do this? Perhaps we can offer an alternative solution that doesn’t involve such a radical action.

Why do you want to do that? What year is your Mac? What macOS version are you running?

Cheers,

Jack