You will find information on how to apply for a visitor's visa on the Norwegian Directorate of Immigration's website.
How to apply for an extension of a visitor's visa
If anything happens requiring you to prolong your stay in Norway or the Schengen area beyond the expiry of your visitor's visa, you can apply to extend your visa.
Do you have a visitor's visa (Schengen visa)?
If you do, you can travel freely in the Schengen countries for as long as your Schengen visa is valid. You must apply for a new visa if you leave the Schengen area and your visa does not cover more entries. Apply at udi.no.