How do I swipe Laptop Hard Drive Clean?

Now, I want to sell my Acer Aspire 5536 as I will be purchasing a new laptop this week. I want to wipe the hard drive and everything clear of anything to do with me. I dont have a reinstall disk for any OS, the intention is to sell to someone who is prepared to install what ever OS they want on there. I just want the hard drive wiped before it leaves this house!!! Can anyone help?