Help!!! I'm over my head

I'm going to Norway in a months time and just decided it might be nice to find some of my relatives to visit - but I haven't the slightest on where to start with my search. I've asked my Grandpa about his mother, who was born in America, and what her parents' names were - he doesn't know anything! Basically, all I know is his mothers name, Anna Vik, and her parents were possibly from Bergen. She was born in Minnesota; does anyone know how to get ahold of a US birth certificate? Cheers!!