Nynorsk, bokmål, or what?

Hello, I have been to a conference about Scandinavian languages, and when it came up to talk about Norwegian the teacher said "I'm from Oslo and I can teach you Oslo's Norwegian... some of my students went to Bergen last summer and they couldn't really understand native speakers... Unfortunally in Norway still prevail local dialects"... this is a bit strange for us abroad cause we want to speak the language of Norway but we just don't know which is the right one (and if there really is a right one...).

Could you gave us some advices?