First find out what the root partition of your current system is.
mount | grep ' / '
Probably it is something like /dev/sda2 when you are not multibooting. I have used /dev/sda2 below, but you must replace that with the correct one!
Use any Linux stand alone CD/DVD. The openSUSE installation DVD is fine. When you use that one, there should be menu entry Rescue.
The boot should end with a console login. It depends a bit, but use root as username and just hit Return when it asks for a password. You are now root in the rescue system. Then
mount /dev/sda2 /mnt
This means that you mount your systems root partition on /mnt in the rescue system.
Then you switch your directory root (not the user of that name, but what your bash session thinks is the root of the directory tree) to it.
Then you fire the passwd commando. It will ask for your new password twice. You will not see anything happen while you type. When the prompt is back, you are done.