There are a lot of helpful posts with good tips for you.
But, one thing that happens that is not mentioned is something that you can think about before you go to the contest.
We all know that if we have a model entered in the contest that we can't judge that category. But, what that is going to really mean is that if you build an aircraft model, it is very likely going to be judged by an armour, ship, or car modeler. Simple logic, since I build USN aircraft, I might have a jet, a prop fighter, and a twin engine fighter in the contest, which means I would end up tapped to judge tanks or cars or ships.
That means that you should concentrate on your actual modeling skills, not the "easy" or "hard" kit, the judges that know that are over in another category, not the nth degree of accuracy, I can name ten aircraft that I want to build for my shelves that the judges would "pick on" for being "wrong", and the models would be 100% "wrong" if built to exactly match the photo
So, if you really want a shot at it, build an Aircraft that the Ship, Tank, and Car guys will recognize,,,,,,and Judge it yourself, at home,,,,,look for everything that can be done Wrong on it, and make sure you didn't do any of those things,,,,,,,,,and look in the logical order of the cuts at the show,,,,are your landing gear straight?, is there a thumbprint in the paint, orange peel in the paint, a seamline along the top of the model, wings and tail at proper angles, fog in the canopy, did you hand paint a canopy on an airbrushed model (uniform level of finish and detail is one of the judging criteria), do you have a resin set of landing gear bays, a resin insert for the gun bay, but, blobby nasty kit bombs hanging on the pylons (uniform level, again)
and always keep in mind that you are building an aircraft for a tank guy to judge, so, even though the USN used gloss overall colors in the seventies,,,,,,,give it a semi-gloss overcoat so that the judges won't think it is "too glaring in the lights"
just some things to think about, from a guy that won't ever be judging a Naval aircraft at a contest
and remember to have fun,,,,,,,and learn what all you can from your first few times, then you will know more for your next time
Rex