Like Brad says ... be prepared - and show up for your squad on time.
Make sure your safety is "off" and the barrel selector is in the right place, glasses on, ear protection in place, be READY to shoot when it's your turn to do so.
Once on the line, concentrate on YOUR TARGET, eyes WIDE OPEN. Don't be concerned with what the other shooters are doing ... the scorekeeper will keep score for you ... you don't have to - JUST SHOOT ! Turn rhe concentration on when you slip the shell in, and keep it on till after the shots been made. Otherwise, relax and have fun in your first registered shoot. No need for nerves, you're only measuring yourself ... not trying to win the whole shebang first time out, so just pay attention to what you're doing and enjoy.