Is the NSA going to take your machine apart and is ready to spend man weeks recovering your data?
dd if=/dev/zero of=/dev/sda bs=1M
or rather (as we are not logged in as root)
sudo dd if=/dev/zero of=/dev/sda bs=1M
Might want to be booted single user, or from another device, but I suspect it doesn't matter (tiny chance the machine would crash before you finished writing, but I doubt it).
Machine disk sda will be all 0s, and unrecoverable. Given the way SSDs work, there probably will be some unreachable blocks, that you'd need a utility to get, but they will not be readable with normal tools.
Given that you don't overwrite data on SSDs, you write data, and the drive erases the area via hardware commands, and then writes your data on the new blank area (BIG oversimplification here), I suspect the overwrite a zillion times thing buys you nothing (and all that is just so the NSA cannot take your drive apart and read it with a scanning electron microscope or the like) as the tech is quite different with a SSD.