Focus on nothing but your horse and trust him/her to do the job. Be confident in your riding and do your best. Don't pay attention to the crowd and forget that anyone is even there.
I have a very cray horse how definitely has show nerves, what we did is took him out somewhere every weekend, and I would show up early so I could lunge him for a while, my horse was very bad and we have to give him calming past and all those things, when we took him out using the pasts he kind of realised that it wasn't that bad and now he is very good going out to comps and it is way more fun now that I don't have to worry about him prancing around, running into horses, rearing and bolting in jump courses ( and yes that's how bad he was 😂😬)
Hope this helps
Ride a show every weekend if you can. My trainer always told me: do it often, until it bores you! Also - never try to "fix" anything in the warm-up. You're there to showcase the horse, not school it.