power off the computer, then power it on. When the white screen appears press and hold Command + S.
when the screen goes black with text on it this means you can let go.
Eventually you'll end up at a prompt that you can type at ending in #
type in: mount -uw /
now pick a user and do
This will set the password for that user. you can set them all if the first one you pick isn't an admin.
reboot to reboot the system
optionally, you can do this:
this will cause the computer to re-run the first run apple setup allowing you to make a new user.