Terminal command to delete everything I've ever created
You don't have to neccesarily delete everything. Deleting everything would give you a hard time getting the computer to start or installing the OS. With Terminal, I suppose that sudo rm -rf / would delete everything.

If you want to start fresh, you can boot into Recovery ⌘ Cmd+R. Use only the ⌥ Opt key if you don't have FileVault installed because it has a root console. 

And then reinstall OS. Also, if you don't have permissions, I mentioned the root console. I am not exactly sure if Internet Recovery also has the console.