A good way to wipe it is to reformat it. That clears everything and leaves you with a blank drive. Seems there are plenty of instructions online to do this.
Unless you used the computer for really confidential stuff (like online banking) you could just delete the files. You may have an app on the computer which will shred them for security if you don't want to risk anybody getting at old photos etc.
Another way of doing it is remove the hard drive and hit it a few times with a big hammer, or bury it in the heart of your next bonfire so the heat destroys its functionality. When I dealt with old hard drives containing classified material, I am pretty sure a sledgehammer was employed in their destruction.