information on ancestors

I'm looking for information on my great grandpa and his family when they lived in norway. Names: Peder and Marit Rasmusen. Their children: Rasmus Pedersen(my grandpa)and his 2 siblings: Pethra and Ole. How can I find out who Peder"s parents were;when Peder was born and where? I heard, as a child, Rasmus was a shepherd before coming to the to the USA. Any info would be deeply appreciated Thank you jj(Juanita)