The closest I've come to having to have The Great Poly Talk was telling my mono boyfriend of five years that I was attracted to a girl I'd met. I was testing the waters... but never actually knew what poly was back then. He was absolutely fine about it. But then, he had the view that 'girls are ok; just don't date men'.
The woman I am now in my poly V with had a different Poly Talk situation. She had been with her husband, mono, for about 10 years. They were having a lot of difficulties and she basically issued the ultimatum - poly or divorce. Not sure that's the best way to go about it either.
I'd say that of course it's normal to feel nervous. I'm betting you feel about as nervous as I did telling my family I (thought I was) a lesbian when I was 13. Then about as nervous as I felt doing a U-turn when I was 17 and telling them "oops, I meant bisexual"... haha....
I would just sit your partner down and take the plunge. Once the words are out of your mouth, the hardest part of talking will come. It will flow from there. Hopefully, they will have questions about your feelings.
I think it's completely normal to be nervous... good luck!