Here are a few ways to securely erase your hard drive:

DBAN (Darik's Boot and Nuke) – Use this free tool that overwrites data multiple times, making recovery impossible.
Windows Secure Erase (for SSDs) – If you're wiping an SSD, use the manufacturer's secure erase tool (e.g., Samsung Magician, Crucial Storage Executive).
Command Prompt (for HDDs) – Run cipher /w:C: to overwrite deleted files on the selected drive.