I have installed Ubuntu using the GUI, giving myself a password and everything. I do not intricately remember the process. However, what worries me is that I don't know the following password:

$ su

Password: <the only password I've ever created on this machine>

su: Authentication failure

I just don't know what to do. I'm not in trouble, but I just want to know what's going on here. I can also lock myself out of directories:

starkers@ubuntu:~/Desktop$ mkdir foobs

starkers@ubuntu:~/Desktop$ sudo chmod 777 -R foobs

sudo: /var/lib/sudo writable by non-owner (040777), should be mode 0700

[sudo] password for starkers: <the only password I've ever created on this machine> 

starkers@ubuntu:~/Desktop$ cd foobs

bash: cd: foobs: Permission denied

I'm just a bit confused. How can I lock myself out like this? I think sudo is the key command here. But I'm making the foobs file as open as it can possibly be via the chmod, so why does it lock me out?