Everything that's been said is very true. i'm a relative newcomer myself and was told to "relax and enjoy it" which, of course, i couldn't do at all for my first 10 or so comps.
So here is some "do this - do that" type of advice.
DANCE IN TIME WITH THE MUSIC.
Don't start off on beat 2 in the slow waltz. Assuming you're starting in beginner grade, the judges will know you're not experienced, and will look for good basics. Dancing out of time is a serious basic flaw. I found i had to make a conscious effort to listen to the music to pick up the beat.
KEEP IT SIMPLE.
simple steps danced well look MUCH better than fancy stuff danced badly.
SUPPORT YOU PARTNER
who will also be very nervous
And of course -
LOOK AS IF YOU'RE ENJOYING IT - and try to enjoy it!! At least SMILE.

Only experience will allow you to dance with confidence and assurance. there are no short cuts. Dance in as many comps and as many events in those comps as you can. When i started as a ballroom beginner, i was also allowed to dance in novice (that's the rules here in Ireland). I soon realised that if i learnt a tango routine, i could also gain experience in over -35 comps dancing against much better dancers, so i did this to gain experience. We came last every time, but it didn't matter - it was important and useful experience.
Good luck in your dancing career