I want to sell my old computer but need to remove all stored files and personal information. How can I do that?

The short answer is to format the hard drive. Usually this requires booting from a bootable CD or USB stick that contains tools to format the hard drive. Some computers also come with a recovery partition on the hard-drive that you can use to do a fresh install of the OS, but this won't guarantee your data can't be recovered if someone has the right tools to do it. Either way, you'll have to interrupt the computer's boot process to tell it to boot from the alternate media. Even better is to get a program that will overwrite the hard-drive with zeros, to ensure the data cannot be recovered. This will also make the computer unusable until a new operating system is installed.