You are very young to go through this with your grandfather.
This isn't going to be easy for you or him. Most hospitals have cancer support groups; you should look into one. If your against them for yourself, going to some might still help you learn things for him. How to help him cope, what kind of things to expect if he's going to undergo chemo.
If you are enrolled in regular high school, you should make your counselor/teachers aware of his illness. This way if you need "exceptions" for assignments or tests or anything, you've mentioned it ahead of time.
Whatever happens, always remember that none of this is your fault. Pancreatic cancer is very asymptomatic, you can have it a very long time and have no obvious "symptoms". Most of the time no one see's it coming; and them BAM!!
Support your grandfather as best you can, and cherish your time with him.Just don't forget to find some type of support group that can help support you. My guess is most of your friends won't understand what your going through.