Biggest one is don't fret getting all the rules right on your first time out.

Just focus on remembering the fundamental rules, and if you're not sure on how something is supposed to work in the rules, make something up that's fun and keep the game going.

Donovan is spot on.

Some things that have helped me:

1. Have the dueling manuevers printed out for each duelist in the party and one for you. It makes it easy to reference them during a fight. 
2. When in doubt about what to do reference the characters' stories and your plot backbone.
3. What's a plot backbone? It's what the Villian is planning to do before the PC's get in the way. 
4. Have fun! Worry about getting the rules correct for the next session. At the next session, tell the players upfront what happened that wasn't correct, and take a moment to explain the correct rule. BUT don't Retcon anything that happened, just play with the correct rule going forward. OR, See page 18 in the Corebook and use your House Rule. It's your game. Rulings not rules. :)

I tell my players that the GM's job is to referee entertainment at the table to avoid things getting out of hand causing to misery to those at the table.

So yes, rules are important, but during your first session do not let those rules overwhelm you the GM, your Players or the entertainment value at the table. 

During my first 7th Sea 2e playtest, one player "Munchkinged" our the Duelist, Ennio Vespucci, and cut through all the Brutes faster than I thought possible. I did not have a quick reference to pull him back, so since the whole table found the antics entertaining, we let the specifics on Dueling slide by.