If it is the only drive in your computer, then the easiest way to do that is to burn DBAN to a CD and boot from it. It will issue a command to the drive to erase itself.
Alternatively you can simply full format the drive. Either by booting from a CD (e.g. from your windows CD), or if it is a second HDD, while booted from your other drive. A single full format it enough to clean the data for all normal civilians.
If you want to sell it to someone who mights throw a small mountain of money in dedicated hardware against it (such as the NSA), then you want to do more then just format it once. But again normal people with no special hardware a single full format is enough.
[Edit] As reference to the last, see:
Peter Gutmann, www.anti-forensics.com and our SE site on security/
All show that a single format is not enough if:
- You use old style platter from 1996. (Modern disk use much smaller marking. Not for security but for performance reasons).
- And you overwrite a disk with all zero (typical for a classic format)
- and you have a scanning tunneling microscopy
Then you can recover data from a disk.