what does “vær så god” mean?

It directly translates to “be so good,” but it means “you’re welcome” or "be my guest." It’s what one says before dinner when inviting everyone to the table. It’s what one says when thanked for good food. It’s a polite way of saying, “After you,” when bumping into someone while you’re both trying to go through a door.

how do i say it?

It sounds like “vah-sha-GOO.” Click here to listen to that and several other typical Norwegian phrases.