https://support.apple.com/en-us/HT201065
Just follow this. On flash storage device like yours the secure erase options in Disk Utility will be greyed out. So an alternative is before you move to step six on that web page, turn on FileVault encryption and let it finish encrypting. Then go ahead and proceed with step six to erase the disk and reinstall the OS.
When you get to the steps where you are in recovery to reinstall the OS, first thing you do it open Terminal from the Utilities menu in the recovery screen. Then enter the line below in Terminal exactly like I have it, including the quotes. That will remove the entire encrypted volume.
Code:
diskutil cs delete "Macintosh HD"
Then quit Terminal and use Disk Utility to erase the disk. After that quit Disk Utility and reinstall the OS.
So what you have done here is encrypted all your data, then deleted the encrypted volume and also formatted the drive. So even if somebody were able to use recovery tools to recover what you formatted, it will still all be encrypted and completely inaccessible.