In some cities like Trondheim, Oslo and Bergen, there are alot of students, and it is harder to find accommodation. So start looking early! Most ESN sections do not provide accommodation. Ask your international office about help and tips.
In each city there is something called "Studentskipnaden" (Student Welfare Organisation) and they provide cheap accommodation for students.
For student dorms in other cities, check out Boligtorget.
You can also look at the private market. The most common pages are:
Finn is used for all kind of accomodation and is the page with more options and hits. If you want to make a post with a description of what you are looking for so that everyone can find it, you have to pay a fee. Finn is the biggest search engine used to find tickets, hotels, used and new card, bikes, furnitures, etc. You can also find stuff that people are giving away for free.
Hybel is mainly used for finding rooms for students. It is free if you want to make your own post that you are looking for accomodation.
Other ways to find private accomodation is to post in groups on Facebook for the university you are attending to see if there might be Norwegian students renting out their places. More and more Norwegian students are going on student exchange abroad for one or two semesters and want someone to take over their room until they are back.