Watch youtube videos of world caliber performances... APHA, AQHA, and Arab in all the major events, for us it was halter, hunter under saddle, trail, western pleasure, reining.
Study the winning rides and winning horses, and get your eye trained for superior movement.
Make a toolbox of phrases to use in your reasons, and practice, practice, practice in front of your team, parents, and friends. That way when you are nervous, you have the rote memory to just relay the appropriate phrase ... "the big bay was the most consistent mover in the ring, moving obediently and correctly. It was noted he maintained his collection on a loose rein, driving deeply from behind with smooth transitions both ways of the arena, requiring no checking or fixing from his rider."
Practice your stance, my girls stood squarely, with legs slightly apart, and hands behind their backs (which decreases fidgeting). They looked confidently at the reasons taker, and started with "Good Evening Ma'am (or Sir)... In the class of Western Pleasure, I found an obvious top horse in the bay, a close middle pair in the sorrel and palomino, and a bottom in the black, resulting in a placing of 1, 4, 3, 2." Emphasizing One, FOUR, three TWO.
Halter emphasize femininity in mares, balance, structure, and muscling. Cadence and collection is important in pleasure, length of stride in hunter under saddle... etc. Your horse judging packets should have a collection of phrases for your toolbox, internet searches on horsejudging will give you more. Find the ones you like, memorize them, and apply them in the classes you watch.
You can make up points in reasons even if you place the class differently than the judges, if you give a compelling reason why you placed the class as you did, so reasons are what will separate good horse judging teams from great ones.
Good luck and have fun, my daughters really enjoyed their time in it.