Hold the Option key while powering on your Mac, this should bring up the Startup Manager.
Select the Recovery disk (10.9 Recovery?) and press return.
Select Disk Utility
Select Macintosh HD (or your volume if you renamed it) from the list on the left
Click the Erase tab in the right portion of the window
Click the Security Options button towards the bottom of the window
Use the slider to determine how secure of an erase you want. The "Fastest" option is obviously the fastest however it does not securely erase data. The next fastest option uses one pass of writing zeros to remove all of your data. The next fastest option uses two passes of writing random data followed by a pass of writing known data to destroy your actual data. The slowest, but most secure option is the same as before but makes seven passes. Depending on the size of your disk the time difference between these could be huge. The most secure option could take anywhere from 3 - 18 hours in my experience. Either of the previous two will take a good bit less time while still providing reasonable enough security if the Mac is returned to a reputable retailer.
Once you've chosen your security option click OK.
Click Erase.
When the process is done the Macintosh HD volume will be blank. You can either install Mountain Lion / Mavericks back over it using the Install Mac OS X option on your recovery disk or you can leave it blank.
