Good luck!
I normally judge the first round and then the finals at the NHC, but this year I'm going to be missing all of the first rounds that I"m near. The first round that I would go to normally is in Indianapolis (last year Chicago), but it's early this year and I won't be home from Texas yet. The first round in Texas is after I leave Texas to go home! So this year I'll just be judging in Minneapolis.
There are a couple of things different about the NHC. As you know, you'll get scoresheets from the first round and if you advance to the second round, you'll send more beers in. It's a great feeling to win the National, for sure! However, for many people who don't aspire to win but want good solid feedback on their entry, sometimes a smaller but well regarded competition is better. You'll get better scoresheets (generally speaking), because in the NHC we use "checklist" type of score sheets at least in the final round.
I've been judging since 2006. What advice I'd give to to remember that your beer will be judged strictly to style guidelines so make sure that you've nailed those guidelines. What I mean is this- say you brewed the world's best beer. It's perfect in every way, but doesn't fit into the style guideline you entered it in. Say you entered it as am amber ale, but it's got some roast to it that tastes awesome. Well, it will probably score very very poorly, because an amber ale should have caramel notes, but no roast, and mild to moderate hops. So many times I've tried a beer that has been GREAT, but scored it in the low 30s or high 20s, because it did not represent the style. So make sure it fits.
The other thing that can happen with a brewer entering a comp for the first time is the expectations. First, a very good beer will score 30-37, while an excellent will score 38-44. Above 44 is world class. (This is out of 50). Few beers, even commercial beers, are "world class" or even "excellent". So if you score, say, 34, that's a good score! But many brewers think they have a 50/50 beer and get angry when they don't score well. A quality competition will give you a scoresheet on improving the beer, the flaws they detect and how to correct them, and tell you how you may have missed the mark. For the scoresheet we use, see this:
http://www.bjcp.org/docs/SCP_BeerScoreSheet.pdf
Some brewers have Ugly Baby Syndrome. You know, like parents who think Junior is the most beautiful child in the universe but the rest of the world sees an ugly baby. Brewers will do that- have UBS over a mediocre beer. If you and your friends love it, that's great but it doesn't mean you have a 46 point beer there. High scores are rare, so make sure your expectations are reasonable. It's so disappointing to not score as you might have hoped, but it can be a great learning experience!