You want to discard your old computer and want to securely erase the data

from your hard drive, what can u use to do rhis and what is thw process called

The process is called "wipe" or "wiping". There are various methods, you can do it manually and erase file by file, or automatically delete all files. The third way is to destroy it physically. Magnetizing, and breaking the hard drive up is the only way to be 100% sure that someone can't extract data from it.