[Homer]Wheres the any key?[/Homer]
As said, you need a low level format that can fill the drive with 0/1s. 1 pass is enough to stop most people getting at files, several passes makes sure.
Usually it's enough to remove all the OS partitions and create from scratch. Depending on your laptop, caution should be made, as its likely the recovery partition is stored on the main disk.
I would personally, in your shoes with your limited knowledge, learn how to boot into the recovery partition and just install windows from scratch.