First, take a deep breath.
You are 19. You don't have to know everything at once!
Second, do nothing but read for a bit.
Then have lots of talks with your fiancee about what your mutual goals are.
I don't mean just the poly ones -- but school, work, where to live, etc. How these things figure into your lives.
The ability to love more than one is polyamory.
That 4 person thing -- you sound like you are talking about a quad configuration of some kind.