How to completely delete my hard drive?

I am going to sell it hence I want to be sure everything is gone. i don't have a Windows 7 disc (the one I use on this computer) to reinstall the software, what should I do, for I fear there won't be any operating system any longer hence almost useless. I am sorry if these are stupid questions but I know... show more I am going to sell it hence I want to be sure everything is gone.
i don't have a Windows 7 disc (the one I use on this computer) to reinstall the software, what should I do, for I fear there won't be any operating system any longer hence almost useless.

I am sorry if these are stupid questions but I know almost nothing about computer apart the basic.

Thank you if you could be quite specific with your answers.
7 answers 7