Well first of all we need to know whether you forgot the username of a normal user account or did you forgot the root password?
- If you know the root password, resetting a normal users password is quite easy, just open a terminal and type;
passwd username where the username is of course the user you wish to reset the password for.
- If you wish to reset the root users password, that will be a bit trickier but here are the instructions for that as well;
Recover Root Password - openSUSE