What I've noticed throughout my walk/trot crossrails experience is that judges want to see that you are having fun and enjoying your time showing. Make sure that you are not too nervous, remember that this is supposed to be fun! Don't overworry about how you look or make problems that aren't there (especially negative predictions of what might happen in your round). Just get out there and enjoy your first show!
Diagonals are important but so is staying in your space, if you have a big strided horse and you get up on other riders the judges will count you off for that. And just stay relaxed when you're doing eq, it's all super fun as long as you aren't worried about anyone but yourself and just ride without thought.
I don't ride english anymore, but I used to. I still have tips from when I did english. Keep a smile on! It's ok to be nervous! I'm still nervous when ever I do the barrels! Sit straight and make sure you have the right diagonal! The diagonal is always important! It's ok to mess up a few times! Everyone has a mistake! Check your diagonal every now and then!
Stay calm, and remember you will be judged on your body position, and confident attitude. In the jumper ring I like to speak to my horse the whole time, so he and i are always on the same plan. this may help you, but be careful as their ears may turn back to listen to you, and some judges may fault that.