So you want to delete everything off the hard drive but the operating system? Its possible, but a pain in the ***.
You have to first un-install all applications and games, except for the ones that come with whatever operating system you are using (you didn't state which one). Then you need to delete all temp files. Then delete anything in your profile's folders, like in "my photos" or "my music", etc. Then, you need to delete all your browsing history, temp files, cache files and cookies. Then, you need to remove any personal preferences, and profiles. Even after all that, everything you deleted is really not "deleted", but the spot on the hard drive that information occupies is marked as "available" by the hard drive's file allocation table. Anyone with a small amount of computer experience can go in and un-delete all that stuff.
If you want to do this by hand, follow the steps outlined above. I can't tell you exactly what to delete or even what the paths or folder names are, since every computer is different, and you didn't mention what OS you are running. Once you delete everything, then run an application like "Eraser" to completely wipe all free space on the drive. Only then will the contents really be "deleted" permanently.
If you are unsure how to do this, get someone to do it for you. Or, you can go the easy route and use "DBAN", then re-install the OS.