There are several ways to secure-erase your hard drive (e.g. SATA or SSD) before selling, gifting or otherwise disposing of it. Here are a few methods:

• Using the DISKPART command from a Windows command prompt that was opened with “Run as Administrator” option (this method of erasing is not as secure as the next methods),
• Purchase PARTED MAGIC (about $15), download and burn it to a USB drive or CD, and perform a “secure erase” or “enhanced” secure erase, or
• Download and use DBAN, which is Darik's Boot and Nuke, a free utility that you can download as an ISO file, then write/burn the ISO to CD, DVD or USB drive to make a bootable medium, and boot to and use DBAN to wipe single drives or multiple drives simultaneously using either Department of Defense 3-pass method (totally adequate) or the DOD 7-pass method. The easiest way is to boot to DBAN and then run it in interactive mode, as explained below.

Writing/burning an ISO file to create a bootable USB drive could be done using RUFUS, a free utility. You can find your particular version of RUFUS here https://rufus.ie/en/ (the easiest way to select and download your appropriate version of Rufus, and some instructions for its use) and here http://rufus.ie/downloads/ (the more complex page to select your appropriate download, and without any instructions).

Writing/burning an ISO file to a CD or DVD could be done using various applications and utilities, such as BURN AWARE FREE found at https://www.burnaware.com/ and the download page https://www.burnaware.com/download.html. Please understand the difference between downloading the FREE VERSION of BurnAware versus the Free Trial of the paid version of BurnAware.

You can download DBAN Here: https://sourceforge.net/projects/dban/

If you buy Parted Magic, this describes how Parted Magic performs secure erase: https://partedmagic.com/secure-erase/

To securely erase a disk involves overwriting all disk sectors multiple times with anonymizing data, such as writing zeros to all sectors on the first pass through the entire disk, then writing all ones on the second pass, and writing randomly zeros and ones on a third pass (or overwriting with other secure sanitizing sequences, such as 7 passes rather than 3).

DBAN has its own boot environment (no need to install it on an existing operating system) so you can use it to securely erase the boot drive or other data drive on a computer without first removing that drive and installing or attaching it to another PC. Caution! Be extremely careful when using DBAN so that you don't accidentally wipe the wrong drive.

1. Download the DBAN *.ISO file here: https://sourceforge.net/projects/dban/
2. Write the ISO file to either a USB Flash drive, CD or DVD so that it becomes bootable (DBAN only requires approximately 20MB of disk space). See the discussion above about using Rufus or BurnAwareFree to write the DBAN ISO file to your preferred boot medium.
3. Boot from the DBAN USB, DBAN CD or DBAN DVD drive. DBAN loads a menu having a blue background.
4. Press the “Enter” key to start DBAN's interactive mode. DBAN will search for and detect your drives. Another menu will appear on screen which shows all of your drives and various options.
5. Select the drive(s) you wish to wipe. Use {up-arrow} or {down-arrow} keys to move up and down the list of drives (or you can use the J and K keys to move up and down) and then press the spacebar to select the drive or multiple drives that you wish to secure erase. Each drive that you select with the spacebar will now show “wipe” next to it. You can toggle the spacebar to de-select a drive mistakenly selected.
6. Select the method of drive erasure. The Department of Defense “Short Method” (3-passes) is selected by default. However, you can select a different method, including the Department of Defense enhanced method by select the “M” key. The DoD short method is a 3-pass version of the American Department of Defense wipe process. It overwrites all sectors with zeroes on the first pass, overwrites them with ones on the second pass and then uses a random pattern on the third pass. Another standard Department of Defense method is DoD 5220.22-M erasure which involves making 7 passes. The more passes that are made in accordance with the chosen secure erase method, then the longer the secure erase process will take to complete. The DoD short method should be just fine for most purposes. The 7-pass method would seem to be rather redundant. But, it's your choice.
7. Press F10 to start the wipe / secure-erase process. Depending on the number of passes, the capacity of your drive and its speed, the wipe time can vary between minutes or several hours.
8. When the wipe process is complete, DBAN will display a message that it has wiped all the drives you selected for secure erase. It should now be safe to sell, gift or dispose of your drive without worrying whether someone could recover deleted files from the drive.