Before SafeErasing your entire computer you will need to confirm various security prompts. Follow the instruction in the dialogs. You can cancel at anytime to avoid the unnecessary deletion of your computer.
Please bear the Warnings in mind and confirm the security prompts if still sure. When the computer reboots all data will be SafeErased this can take up some time. Please do not turn off the computer during this process, otherwise the deletion process might not be completed correctly.