Disk Utility has a secure erase function.

1. Open Disk Utility
2. Select the disk that you which to erase
3. Select the Erase tab
4. Click on the Security Options… button
5. Move the slider to Most Secure
6. Click the OK button

This screen shot is for Disk Utility, version 13 (517), that comes with Mavericks (OS X 10.9.5), later versions may vary in appearance:

This will securely erase the disk. As you have selected the most secure option, this may take a while.

This option states:

This option meets the US Department of Defense (DOD) 5220-22M standard for securely erasing magnetic media. It erases the information used to access your files and writres over the data 7 times.