To be able to work in Norway, you need a working permit. EU Citizens can apply online through this link at the Norwegian Directorate of Immigration's webpages. You have to log on with a username and password, and then fill out the form "job seeker". If you come across any difficulties when filling out the form, feel free to contact Heine Nilsen or Yngvild Hjetland at the Human Resources Office at NHH.
If you are a non-EU citizen, you could either apply through the Norwegian Embassy in your country, or NHH could apply on your behalf (reccommended, as this usually takes shorter time). You will be contacted by the Human Resources Office at NHH and then given the needed information in order to apply.
Among the first things you should proceed when you come to Norway is official registration for a Norwegian ID-number, tax deduction card and residence permit. The Human Resources Office will help you with the process. You can read more about the process at Euraxess' pages, but please notice that in Bergen there are no Service Center for foreign workers. At NHH the HR office will help you get started instead.
For more general information on visas and work permit, please check out the Euraxess' webpages.