Here are some things to do to prepare:
-print out references. If you can print out different views of the character (front, back, sides, up-close details), all the better.
-bring in-progress photos with you to pre-judging, as well. This shows that you made the costume, and judges enjoy seeing the construction process and some details may be hard to see underneath layers and layers even with a lot of poking and prodding (for example, if you made a corset to go under your costume).
-if you think you'll get nervous and forget what to say, write down on an index card a basic run-down on the construction process - materials, methods, things you like most about the costume, etc.
-never point out what you don't like/did wrong. Not only because you're drawing attention to a flaw, but judges don't like negativity.
Go into the prejudging room with pride in your work. This is your first contest and you'll be judged most likely in the Novice category (some cons don't have categories...I see you are from Florida, and I know down here it can really vary from con to con), but the judges should know you're new to cosplay and won't judge you nearly as harshly as they would a Master level craftsman, anyhow. I know it can be hard to smile when you're feeling nervous, but like Sarcasm-hime said, your average judge is not going to be mean in any way, shape, or form. I'm not going to say all judges are nice, because I've had my fair share of ones who were not - but no judge should ever give you a critique during the process of judging.
2014 - Upcoming Conventions :
Holiday Matsuri (FL)
Upcoming costumes : Suzie Q from JoJo's Bizarre Adventure, Edea Lee (various versions) from Bravely Default, Robotnik (genderbend) from Sonic, and more!