Boot into Runlevel 1 AKA single user mode and You can add/change the password from there.
For Ubuntu-based distros that's the "recovery mode" boot menu entry. (You might need to tap the escape key before booting to bring the boot menu up if it is configured to bypass the menu and instantly boot the first menu entry.)
When in the single user mode, run this:
passwd [your_username]
it will ask to enter a new password and to confirm it
then, run this command:
reboot
(this one should be self-explanatory )