If you are from a country outside the EU/EEA and wish to work in Norway, you need a residence permit. If you do not already have a residence permit, you must apply for a residence permit for work.
If you are an EU/EEA citizen who is going to work and live in Norway for more than three months, you must register with the UDI - The Directorate of Immigration.
Nordic citizens do not need a work permit or UDI registration.
The UDI provides detailed information on their web pages.