Absolutely no reason to be nervous! One of the best things about brewing competitions are getting to see the judges comments and helpful tidbits of advice. They can really help you learn and brew even better. I've judged many an event when the wiz-bang zillion dollar brew rig 20 year brewer, entered pure crap, and the brand new pot-on-the-stove extract brewer took home a ribbon.
Follow the instructions for entering and getting your entries to them. Fill out whatever forms, pay the entry fee hoIf you have questions, ASK, they will help you understand their process. Brew your best and enter up!
The best advice I can give is to read the BJCP style for which you are going to enter (category and subcategory), because that style is what it will be judged against. If the style says "low hopped" don't brew 90 IBU's. If it says "pale to golden" don't enter a dark in that category. Simply be familiar with what the judges are expecting.
We judge based on a few basics that are well documented (those guides are available to brewers and judges). Each of the following below has a complete description in the guide for every style. Low means it is not even close to the style. High means it's dead nuts on style.
Aroma 0-12 points
Appearance 0-3 points
Flavor 0-20 points
Mouthfeel 0-5 points
Overall impression 0-10 points
Honestly, i've entered competitions where I knew some high power brewers were entering. I told my wife "those guys write books, do podcasts, and have been at this many years longer than me, I have no hope on this one". But I came home with a best of show award. I've also at times felt very good about an entry and thought it would kick butt, and got a bunch of low scores. No biggie, I paid $5 to get a ton of critique and help, hell yes! Another thing that ive done is bottled entries for two competitions (from the same keg) and entered both. Averaged high 30's in one competition, and low 20's in the other competition. Do not take anything as gospel or take it personally. Judges are human.
Hope this helps calm the nerves