When you boot into grub, select whatever option you use normally to boot. Instead of pressing enter, press e. Find the line that contains the name of your kernel, it should say kernel and then a path and press e again. Insert the word single after root=/dev/whatever and press enter. Then push b to boot. This will boot your installation in what is known as single user mode, with no GUI.
You may have a password on your bootloader. I am not familar with how SuSE works in terms of it's boot loader setup. If this is the case, I believe you will have to press p and enter your password before you can follow the above instructions.